Side remark: On every bowed instrument (such as the violin), when moving from a string to a string, or when changing direction, it is standard technique to briefly stop the bow on the (new) string and only then start bowing again. If a beginner ignores this, they are rewarded with a harsh or scratchy sound as the string and the bow are trying to phase-lock with each other. With practice, this discontinuity can gradually become zero duration (yet present) and completely unnoticeable.

Thanks @JirkaHanika! Having never played a bowed instrument where continuous motion was possible, I had no idea. It sure looks like it's continuous, but that must be the effect of skill -- as you said, approaching zero duration. TIL.

Bows for mainstream stringed instruments (violin etc.) are concave to provide a longer, somewhat stiffer hair surface that makes it easier to play long, sustained notes. Convex bows were used through the baroque period but when louder and more fluid styles of music gained in popularity they gave way to what we see today. I know nothing whatsoever about bowed psalteries but I suspect you might find similar forces at work there.