Thank you for the extra video. I see the hand stays in one place, in closed position, on white keys.
I started working on trills and especially extended trills two days ago. If I tried any kind of speed eventually I'd get jerky and close to cramping. (Extended trills can last a whole bunch of measures). I looked at all kinds of info on the physical side of trills. Among the things I found was:
- don't raise too high above the keys, or maybe, don't let the keys lift all the way
- Fitch had a thing of finding the keys' "sounding point" and only raising that high. My Kawai DP has a construction that tries to mimic a grand with an escapement so I could do that.
- Fitch also suggested "walking" the hand a tiny bit so that you are not stuck in one place, and this stops the system from locking up.
- another source talked of feeling the keys with your fingertips and the feedback does something to your nervous system.
I played with all these things. My trill became effortless, sustained over time, and almost too fast. My hand felt as if it was "melting". Now - when I look at that device, it might block me from feeling the depth of the key, or it's upward push after release for riding it. I don't know what happens with the gentle "walking forward and back", the little movements in the arm, to keep it relaxed.
Having just had that experience, I couldn't help thinking about these things when I saw your post and this idea.