The change of bow direction:
There is no avoiding the bow change, only minimizing the effect of it in sound. The elements we have with which to address the change of bow direction are bow speed, pressure on the string (weight), angle of the bow in relation to the bridge and the string, and proximity to the bridge/fingerboard. Of these characteristics of bow usage, the first 3 are most relevant to the bow change. The “whipping wrist and fingers” that many teach has the tendency to speed up the bow and change the relationship of the bow/string angle in less than expert hands. If this is done with perfect control of the release of pressure it can work. But in my 50 years of observation, it is often not well done and often makes things worse. Of course, in the hands of a brilliant musician like Leonard Rose, this is extremely effective. Sadly, few of us are so extraordinarily gifted.
In my opinion, the less unnecessary motion of the fingers, wrist, and arm at the bow change, the better the outcome for most people. Lightening up the pressure/weight around the instant of the bow change and flexibility of the joints is a useful in this regard. There will always be some reaction of the fingers and wrist, but if this does not interrupt the plane of travel of the bow, the bow angle, etc., it is not a problem.
I was at a violin masterclass with Broadus Earle many years ago when he addressed bow changes. He made the point that even light shined on a mirror stops for a minuscule amount of time as it changes direction. Can we see that? No. How does that relate to the bow change? Like light on the mirror, the bow must stop as it changes. We cannot alter that fact, but we can train ourselves to have maximum control of the elements of bowing to minimize this. I have seen many approaches that suggest different motions at the point of change (circles, figure 8s, “shock absorber,” flipping the wrist), which when executed by great players (like Rose, for instance) are impeccable. Unfortunately, most people when doing these things cannot do them well- most of us are not Leonard Rose. The problem seems to me is that in many cases this extra motion causes a change in bow speed, the plane of the bow, pressure and/or direction that actually increases the sound of the change. Practicing slow scales with a drone string can help with bow control. When you play 2 strings simultaneously, the plane of the bow must be consistent, and when done with a metronome, the speed can be observed by comparing bow distribution relative to the beats; this is fundamental to consistent control of the bow, whether you are concerned with bow changes, or shaping a phrase. Once you can play with absolute control of the speed, pressure, and the plane of the bow, you can then vary those elements with greater mastery and more sophisticated musicianship.
You need to be able to sense the string surface and the way it resists moving with your fingers through the bow. Janigro made a big deal of this, too. Some people will say “carve” the sound, and if you really know how to use a knife when cooking, that is a useful image.
Bow techniques (Sevcik)
Orlando Cole and Lynn Harrell
up- and down-bow stacatto
Dvorak Rondo, Popper Spinning Song
David Finckel on bow strokes
Spicatto with Colle
Overlapping Spicatto and Sautille
Colin Carr on bowing