I don’t think we have anything on this specifically beyond what Troy mentioned in the post above. It has come up a bit in various places but no single-topic video yet exploring this in detail. But yeah definitely something we may look at more in the future!
The player that we have interviewed who most notably uses finger motion as the source of the picking motion itself is Martin Miller. We have a new conversation with him coming up where we talk specifically about the motion and his thoughts on how to do it. Joe Stump also does this while sweeping, and we talk about that in the interview as well. Actually Michael Angelo Batio also uses a kind of thumb-index motion for moderate speed playing and you can see some of that in our most recent interview with him as well, particularly in the Rainforest outro tune.
I would caution that fingers are complicated and lumping everything under “finger motion” is a thing we tend to do because it’s not something we’ve really looked at in any great detail. What Martin does looks pretty similar at times to what Joe does, and to an extent similar to what Batio does in that the entire index and thumb appear to move together.
As to what other kinds of finger motion are out there, and what kinds of lines can be played with them, that’s still an open subject. I suspect just like all the movements, some types of finger motion will be pickslanting movements and will only really work on certain types of phrases. And others, like Martin’s, are used to create a completely escaped pickstroke and can be used to play lines that require that, like 1nps arpeggios. And as always, I’m sure arm position plays a role in determining which of these various capabilities the movements actually have.
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I’m a member at his school so I asked him questions about thumb position (mainly contracted at lower strings E6 A5 D4 and extended for higher G3 B2 E1) and his plain answer Was ”tone, it’s for tone. The thin strings sounds better with lesser angle”
In one way it’s true, I don’t personally like the tone with a steep pick angle for the higher strings, but there’s more to it. It also serves as making string switch easier because when you slightly extend thumb (Going down, vice versa for the other way) for each string you help your wrist/arm with the string switch. And everything counts 
Btw: still waiting for my magnet so I can analyze myself!