I just noticed that there's a delightful bonus when I am at liberty to bend my thumb. I can phrase. I have somewhere to land after an up feeling. I am not up all the time. I can come down, musically and physically.

It works both ways. If I bend my thumb, it helps engender the downward downbeat arrival placement in a timely fashion. And if I strive to make the consequence of an upbeat feel and sound right, I discover that a locked thumb impedes it.

It seems to assist this when I have the thumb straight (but not bent backwards) during the upbeat. The thumb seems to play the music with me. How helpful.

Another surprising twist is that these upbeat and downbeat thumb responses need not be on upbows and downbows. They can happen anywhere in the course of a bow stroke, as long as the music calls for the appropriate inflection. It is incredible, this pliancy and independence of the thumb.