In the first video you spent a lot of time describing the mechanics of the down stroke. What is happening to your fingers and wrist when you subdivide during an upstroke?
Great question, and it illustrates one of the regrets I had about the time limit for youtube. It's hard to describe in words, but I'll try, since more detail is needed on this:
- the fingers themselves don't really do anything different at all in terms of their shape. On both upstrokes and downstrokes, the fingers of the right hand move a bit closer to the palm while rotating against the fulcrum of the fixed thumb. Lynn Seaton has a wonderful and funny demonstration of this where he describes holding the four fingers together and waving them like a seal would with its flipper (Lynn adds sound effects

) . (My take on this flipper is that it includes the wrist and the hand, where I believe that Lynn's is more in the fingers - either way, I got/adapted a lot of this from watching/studying him)
- having said the above, the right wrist actually provides most of the motion on the upstroke, but it's subtle. Rather than having me waste a bunch of words on it, try this:
1) With all four fingers of your right hand, tap slowly on a table or desk in front of you using your whole arm at roughly mm=60. It will make the sound "SLAP".
2) Next, subdivide this in half to where you tap lightly on the upswing of the arm; this should make the sound "tap", so you're basically going SLAP-tap-SLAP-tap etc. Notice how relaxed the "tap" feels as compared to the "SLAP".
3) Divide again into triplets (SLAP-tap-tap.....) and 16ths (SLAP-tap-tap-tap...) until it feels like a natural motion. Notice that at the triplet/16th division levels you really have to pace the upswing of the arm so as not to be done with its upward motion before you have tapped two/three times; this is part of the choreography of the movement that I believe helps the player get more physically in sync with the music.
4) Now get in your normal bass playing position with the more-or-less perpendicular fingerboard, and do the same exercise slapping the fingerboard with a free hand while observing the arm and wrist motion. After this, anchor the thumb and try to recreate the motion against its fulcrum point while plucking the strings. Go very slowly at first, as it's a very precise balancing motion at each subdivision, and a motion that is too big or too small will make the time rush or drag.
1-3 above can also be done by holding the left palm upward and immobile in front of you and clapping/tapping on it if you prefer.
Does that help?