Bow Direction Changes - Circular Motion in the Bow Stroke
As the basic bow stroke becomes more refined, the bow should not travel on a flat plane in a static "back and forth" motion. Rather, the angle will change slightly from downbow to upbow, and the shape traced by the bow hand will be an ellipse.
A good way to introduce students to this concept is to begin with large circular (elliptical) motions. Have the students play string crossings on open strings as shown in this video. Note the bow hand of the cellist, and the elliptical shapes it draws.
Down bow D string Up bow A string
Down bow A string Up bow D string