The finger joints play an important role in violin bowing. They should always be flexible. The hand puts strong pressure on the bow through the index finger and this causes the necessary resistance from below, from the side of the thumb.
But the thumb, even when it presses strongly on the bow stick with its pad, must be extremely soft and flexible, especially in the nail joint. Just as the spring in a car has no effect on its movement, but creates comfort for passengers, the elasticity of the thumb is necessary for a calm and smooth movement of the bow.
In the picture, the straightening and bending of the fingers are exaggerated just for the reason to bring your attention to their opposite positions.
Small active lateral movements of the fingers, but only at the main joint, can be achieved with persistent and diligent practice. It is impossible to obtain a full sound with these movements, because the fingers almost can't move in the same direction as the bow. But their mobility is important to support the actions of the elbow and wrist joints, especially when changing strings softly at the frog. An important function, in this case, is performed by the joints of the little finger: while its finger nail phalanx is pressed against the bow stick, all three joints should bend freely.
Every single joint plays its role in maintaining the move of the bow. However, the most important thing is their coordinated interaction, because there are no strokes performed by just one joint, but there are strokes in which one joint takes the main work, and the rest are helping. Joints are moved by muscles. In the forearm there are muscles that move the hand and fingers, in the shoulder there are muscles that move the forearm, and on the back and in the chest there are muscles that move the shoulder. In order for all muscles to do their work freely, the position of the hands should be relaxed. Any convulsive muscle tension should be avoided. As soon as you tense just one muscle unnecessarily, the freedom of movement of one, and sometimes several joints will be instantly limited, since the muscles are always located one joint higher than the joint they set in motion.
That is why? right from the beginning you have to make sure that muscles of your arms are maximum relaxed, because only in this case each joint will be able to do its work
Every violinist must clearly understand the nature of right arm move and its role in violin bow moving.
Automation of movements can be achieved only through conscious exercise. Mechanical, thoughtless repeat of thousands of bow exercises can only lead to musical dullness.
