Last night I was thinking about the way the left hand jumps right at release. In the end the issue may be the bone structure of my left arm and the way I am holding it in kai. Another kyudo book, 「技を極める弓道」, mentioned that the way to get the most power from the left arm is to have the radius and ulna, the two bones in the forearm, aligned one over the other (that is, in an plane perpendicular to the floor).
I’ve noticed in the dojo that there is surprising variety in the bone structures of people’s arms, especially among women, and the book confirms that there are people who naturally have these two bones twisted in different ways when pushing forward (say, against a wall). So looking at my own arm, it seems that the two bones are not optimally aligned in the way I naturally hold the bow, so tomorrow I’ll try to align them. Some experiments with a gomu-yumi make this seem promising. If I hold the left arm as usual, it doesn’t move much either left or right at release, but if I twist the elbow to align the two bones, the left hand naturally goes to the left at release. It is possible that I am affecting this subconsciously (no way to do a double-blind test!), but if it works that way with the comparatively weak gomu, I imagine the effect will be more pronounced with the real bow.
But maybe too pronounced. The book mentions that this is a very individual thing, so some experimentation will be needed, but I have time, so why not?