OTL. Anyway, time for actual discussion. *Sorry for wall of text rambling, but it's interesting I promise!*
For lazy people, tl;dr version is: difficulty determination should be based on practice time rather than rarity of the trick/linkage, but there's heaps of variables affecting practice time, so best thing to do is practice more to get better personal evaluation xD
Strongest finisher by fel2fram (2-hand interlaced around/twirl fall variation? unsure of naming conventions)
What are the most impressive feats or tricks in PS history? From before 2008: sunrise's perfect thumbspin 15.5 with pencil is probably craziest trick every recorded (I still am unsure how much practice or skill would be needed to replicate it), and cloud traveller's midbak 1.5 x 12 with mx deserves honorable mention. However, before 2008, index bust x 10-15 was considered incredibly difficult, with hunlanlin holding world record of x 17 before spinnerpeem joined the scene.
From peem's erasing of the boundaries of difficulty which everyone else was constrained by, his explore M fl combo and his WC10 finals combo stand out most. From 2010 until 2013, fel2fram rewrote the comprehension of creativity and possibilities in linkages entirely, making many variations and concepts that are still barely understood, much less explored. A13x's dual pass T4, fel2fram's interlaced 2-hand around variation, menowa's spiderspin variations, xound's fingercrosses in WT15 also stand out.
However, consider this: since 2008, why has no one even come close to sunrise's thumbspins, but many many spinners are able to do bust x 10-20 or more (with fair numbers of spinners reaching 25+ or more busts in their first year)? When TEK uploaded peem's video of 50+ palmspin fl ta, other hard trick experts at the time were still in the < 10 range, but it did not take long for spinners who'd spun less than 1 1/2 years to reach 300+ palmspin fl ta (tony from HKPSA and yaoss from MYPSC).
Is the typical 1 year spinner of now better than the experts of 2008? Does the typical 1 year spinner of now have some magical technique of training busts or have more practice time on that trick? Most likely not, but the seeming 'degradation' of power trick difficulty for more basic variations (index bust, fl ta, spread, fl ta palmspin, aerial hai tua, hai tua) is probably associated with world's 'belief' of what is possible and impossible; i.e. if spinners believe certain feats are achievable with certain practice amount, then it contributes to becoming self-fulfilling prophesy (and vice versa, if certain feats are believed to be impossible, then this belief is strong enough to deter people from trying or even increase their actual failing rate). Of course, more available resources (decent quality demonstration videos and tutorials) plays a part, but generally I do not consider the majority of tutorials to be that helpful or well made, and many spinners are unable to elicit maximum details from slow motion videos either.
The key point of contention in difficulty is 'practice time required to master the trick' versus 'how many people can do the trick' (or linkage) in question. Personally, 'how many people can do the trick' is not that relevant when considering tricks of high difficulty, and is further obscured by trends/popularity (e.g. rex trick is quite simple to do, but was considered amazing when rex showed it in WT11 R1; similarly menowa's WT15 R3 combo gained much attention for its angled around variations but is far easier than his WT15 R2 combo or even his WC14 R2 combo). Furthermore, if people believe trick is too hard, few people will attempt it, which reduces estimation of 'how many people can do the trick', leading to overestimation of the trick's difficulty. Therefore I prioritise 'practice time required to master the trick' as being of far greater importance in difficulty consideration than merely 'how many people can do the trick'.
Difficulty can also be considered in terms of spinners' ability to devise techniques for tricks that are not yet in their mastery. This is highly linked to the spinner's current skillset, and the similarity of the trick/linkage they are trying to learn. For example, pinkybak 2.0 cont is quite similar to pinkybak 1.5 cont, so mastery of pinkybak 1.5 will help greatly. This can be applied to linkages and more extreme examples, such as applying fl ta cont technique to do PD fl TA (which allowed me to reach pd ta > pd fl ta x 4 in my 5 year solo with less than 30 minutes experimentation, and increased to pd ta > pd fl ta x 7 in 'THIS IS POWER V5' despite me never devoting deliberate practice for this trick in the past few years). Certain tricks which are more different to existing tricks commonly done are harder to devise techniques for, but may not actually require as much practice time as expected (e.g. dual pass T4, which I managed x 13 of after training for 30 minutes; whereas fel2fram's 2-handed interlaced around variation seems impossible to do smoothly even though perfoming it sketchily is not that hard).
On the other hand, slight variations which are fairly common to mastered linkages are difficult because hand's muscle memory defaults to the common linkage instead of the desired variation (e.g. east sonic and west sonic variations used by iteza - see below video 0:04-0:05 linkage, and hash, slofis' inv shadow+arounds in Pearls 6th combo). Of course, spinner with very good ability to control fingers will not be set back by the defaulting muscle memory phenomenon, but generally even experienced spinners have trouble with understanding and performing these slight variations immediately, so they might take a while to master them completely. However, this raises the question of whether a spinner who was not as constrained or influenced by trends to do common material would find east and west sonic variations etc to be hard (we can probably never fully resolve this question, just as innate considerations of appeal in art and music are influenced the moment we learn language, by every comment and every person we interact with and so on).
Regarding practice time, it is possible to stumble across efficient technique quickly (usually by accident) which allows mastery of trick in short time (e.g. Vora's palmspin fl ta supposedly reached 100+ in several weeks or less, which might be a joke on his part; or nadhif's rapid learning or pinkybak 1.5 and palmspin fl ta). Practice time is also probably influenced by belief of what is possible, as said before. However, practice time varies greatly not just between spinners (even if generally we can come to agreement of what tricks/linkages are hard), but also within the same spinner in tricks he/she considers difficult.
While many spinners (and myself) consider dual pass T4 and say, fl ia - ss cont or fl ma - ring ss cont all to be quite difficult and rare tricks, the practice time required for these 3 is very different. For me, dual pass T4 to 10+ took half an hour, whereas fl ma - ring ss to 10+ took daily practice of 40 minutes for nearly an entire year (both tricks being trained in 2014-2016 period, after I'd spun over 7 years), so clearly there is vast difference in difficulty despite both tricks being rightly considered 'hard'. Of course, maybe someone else would not find the difference as large as I do, but it would require more spinners to master both these unusual hard tricks, which is unlikely to occur to any significant extent.
Of course, placement of trick/linkage in combo impacts difficulty, as doing harder trick/linkage later is more difficult than doing it earlier. Logically, one should practice the hard trick/linkage itself separately quite well before trying to use it later in combo (if you do not, you will have less chances to practice it when recording as you reach later point in combo more rarely than earlier point, and your mental stress will also be increased).
The role of innate finger flexibility and muscle memory in difficulty is also interesting. For example, mastering one or two flush sonic and side flush sonic variations makes learning other linkages in 14 slot far easier. Furthermore, as a spinner with below average middle and ring independence (I cannot make spock sign which separates middle and ring finger properly on right hand), I can still perform most flush sonic and side flush sonic variations in 14 without thumb 'assistance' cheating, so innate flexibility is not necessary for these, even if it is helpful. Also, fact that left hand which hardly trains deliberately can do many 24 and 14 linkages by using muscle memory of right hand reinforces idea that 24/14 linkages are based on muscle memory rather than finger flexibility. Of course, training to acquire the muscle memory and appropriate pen rotation angle/speed/finger movement requires significant time.
In contrast to this, fingercross most likely requires innate finger flexibility (which can be trained by stretching, but risk of injury and takes long, long time) and also muscle memory, but I cannot fully comment on this as I cannot do fingercross of ring+pinky or middle+ring on my right without my left hand forcing the fingers to cross.
Anyway tl;dr version is: difficulty determination should be based on practice time, but there's heaps of confounding factors and other variables affecting practice time, so best thing to do is practice more to get better personal evaluation.
From peem's erasing of the boundaries of difficulty which everyone else was constrained by, his explore M fl combo and his WC10 finals combo stand out most. From 2010 until 2013, fel2fram rewrote the comprehension of creativity and possibilities in linkages entirely, making many variations and concepts that are still barely understood, much less explored. A13x's dual pass T4, fel2fram's interlaced 2-hand around variation, menowa's spiderspin variations, xound's fingercrosses in WT15 also stand out.
However, consider this: since 2008, why has no one even come close to sunrise's thumbspins, but many many spinners are able to do bust x 10-20 or more (with fair numbers of spinners reaching 25+ or more busts in their first year)? When TEK uploaded peem's video of 50+ palmspin fl ta, other hard trick experts at the time were still in the < 10 range, but it did not take long for spinners who'd spun less than 1 1/2 years to reach 300+ palmspin fl ta (tony from HKPSA and yaoss from MYPSC).
Is the typical 1 year spinner of now better than the experts of 2008? Does the typical 1 year spinner of now have some magical technique of training busts or have more practice time on that trick? Most likely not, but the seeming 'degradation' of power trick difficulty for more basic variations (index bust, fl ta, spread, fl ta palmspin, aerial hai tua, hai tua) is probably associated with world's 'belief' of what is possible and impossible; i.e. if spinners believe certain feats are achievable with certain practice amount, then it contributes to becoming self-fulfilling prophesy (and vice versa, if certain feats are believed to be impossible, then this belief is strong enough to deter people from trying or even increase their actual failing rate). Of course, more available resources (decent quality demonstration videos and tutorials) plays a part, but generally I do not consider the majority of tutorials to be that helpful or well made, and many spinners are unable to elicit maximum details from slow motion videos either.
The key point of contention in difficulty is 'practice time required to master the trick' versus 'how many people can do the trick' (or linkage) in question. Personally, 'how many people can do the trick' is not that relevant when considering tricks of high difficulty, and is further obscured by trends/popularity (e.g. rex trick is quite simple to do, but was considered amazing when rex showed it in WT11 R1; similarly menowa's WT15 R3 combo gained much attention for its angled around variations but is far easier than his WT15 R2 combo or even his WC14 R2 combo). Furthermore, if people believe trick is too hard, few people will attempt it, which reduces estimation of 'how many people can do the trick', leading to overestimation of the trick's difficulty. Therefore I prioritise 'practice time required to master the trick' as being of far greater importance in difficulty consideration than merely 'how many people can do the trick'.
Difficulty can also be considered in terms of spinners' ability to devise techniques for tricks that are not yet in their mastery. This is highly linked to the spinner's current skillset, and the similarity of the trick/linkage they are trying to learn. For example, pinkybak 2.0 cont is quite similar to pinkybak 1.5 cont, so mastery of pinkybak 1.5 will help greatly. This can be applied to linkages and more extreme examples, such as applying fl ta cont technique to do PD fl TA (which allowed me to reach pd ta > pd fl ta x 4 in my 5 year solo with less than 30 minutes experimentation, and increased to pd ta > pd fl ta x 7 in 'THIS IS POWER V5' despite me never devoting deliberate practice for this trick in the past few years). Certain tricks which are more different to existing tricks commonly done are harder to devise techniques for, but may not actually require as much practice time as expected (e.g. dual pass T4, which I managed x 13 of after training for 30 minutes; whereas fel2fram's 2-handed interlaced around variation seems impossible to do smoothly even though perfoming it sketchily is not that hard).
On the other hand, slight variations which are fairly common to mastered linkages are difficult because hand's muscle memory defaults to the common linkage instead of the desired variation (e.g. east sonic and west sonic variations used by iteza - see below video 0:04-0:05 linkage, and hash, slofis' inv shadow+arounds in Pearls 6th combo). Of course, spinner with very good ability to control fingers will not be set back by the defaulting muscle memory phenomenon, but generally even experienced spinners have trouble with understanding and performing these slight variations immediately, so they might take a while to master them completely. However, this raises the question of whether a spinner who was not as constrained or influenced by trends to do common material would find east and west sonic variations etc to be hard (we can probably never fully resolve this question, just as innate considerations of appeal in art and music are influenced the moment we learn language, by every comment and every person we interact with and so on).
Regarding practice time, it is possible to stumble across efficient technique quickly (usually by accident) which allows mastery of trick in short time (e.g. Vora's palmspin fl ta supposedly reached 100+ in several weeks or less, which might be a joke on his part; or nadhif's rapid learning or pinkybak 1.5 and palmspin fl ta). Practice time is also probably influenced by belief of what is possible, as said before. However, practice time varies greatly not just between spinners (even if generally we can come to agreement of what tricks/linkages are hard), but also within the same spinner in tricks he/she considers difficult.
While many spinners (and myself) consider dual pass T4 and say, fl ia - ss cont or fl ma - ring ss cont all to be quite difficult and rare tricks, the practice time required for these 3 is very different. For me, dual pass T4 to 10+ took half an hour, whereas fl ma - ring ss to 10+ took daily practice of 40 minutes for nearly an entire year (both tricks being trained in 2014-2016 period, after I'd spun over 7 years), so clearly there is vast difference in difficulty despite both tricks being rightly considered 'hard'. Of course, maybe someone else would not find the difference as large as I do, but it would require more spinners to master both these unusual hard tricks, which is unlikely to occur to any significant extent.
Of course, placement of trick/linkage in combo impacts difficulty, as doing harder trick/linkage later is more difficult than doing it earlier. Logically, one should practice the hard trick/linkage itself separately quite well before trying to use it later in combo (if you do not, you will have less chances to practice it when recording as you reach later point in combo more rarely than earlier point, and your mental stress will also be increased).
The role of innate finger flexibility and muscle memory in difficulty is also interesting. For example, mastering one or two flush sonic and side flush sonic variations makes learning other linkages in 14 slot far easier. Furthermore, as a spinner with below average middle and ring independence (I cannot make spock sign which separates middle and ring finger properly on right hand), I can still perform most flush sonic and side flush sonic variations in 14 without thumb 'assistance' cheating, so innate flexibility is not necessary for these, even if it is helpful. Also, fact that left hand which hardly trains deliberately can do many 24 and 14 linkages by using muscle memory of right hand reinforces idea that 24/14 linkages are based on muscle memory rather than finger flexibility. Of course, training to acquire the muscle memory and appropriate pen rotation angle/speed/finger movement requires significant time.
In contrast to this, fingercross most likely requires innate finger flexibility (which can be trained by stretching, but risk of injury and takes long, long time) and also muscle memory, but I cannot fully comment on this as I cannot do fingercross of ring+pinky or middle+ring on my right without my left hand forcing the fingers to cross.
Anyway tl;dr version is: difficulty determination should be based on practice time, but there's heaps of confounding factors and other variables affecting practice time, so best thing to do is practice more to get better personal evaluation.
