A straight thumb results in straight stiff fingers and a relentless, overly tight bow hold. It also prevents the flexing of the wrist. This lack of small shock absorbers in the wrist and fingers will greatly affect the perfume, nuance and quality of sound, particularly in softer dynamics and classical works such as Mozart, Schubert, and Bach. Carl Flesch famously said that all artistry resides in the small muscles of the bow hand and wrist.
Natural strength and shape of the hand
There is an ideal size and shape for your bow hand which will take advantage of the natural function of your hand and allow a constant conversation between your thumb and fingers.
First make a ring between your thumb and second finger; space the other fingers with whole steps in between. Don’t put your thumb through the bow – find the “captain’s chair” (the little bump on the nut) and bend your thumb as you seat it there.
Galamian believed that the thumb should always be bent at the frog to allow for the maximum space between the thumb and index finger. Put your thumb outside the bow and play at the frog; notice how full the tone is, even though you don’t have much flexibility or control. This is because the space inside your hand is ideal for its strength and therefore your hand can transmit the weight of your arm efficiently. Now replace your thumb in its proper position on the bow, but try to keep the internal space of your hand as large as possible.
The action of the thumb is sideways, not straight up: as the thumb pushes, the third finger answers.
Draw long bows, rolling the bow from the side of the hair to flat hair using only the thumb, not the wrist. Keep an even, good tone.
Subtracting thumb counterpressure at the frog will allow smoother changes; adding it during downbows when nearing the tip will increase contact and avoid diminuendo. (Exercise)
11.3 pounds is the difference in thumb pressure between frog and tip!
Supination can be done in the middle to lower half of the bow, but is especially valuable at the frog.
To supinate, roll onto your pinkie. The thumb and fingers will bend, the pinkie will curve, the knuckles will relax slightly, and the fingers will be square on the bow. Your index finger will slide toward you, releasing the front of the hand.
Notice the change in the contact point of your thumb – there will be a second dent created.