When you are holding the violin, the thumb should not be holding the instrument. Get to know this information when taking violin lessons. The function of the thumb should be to balance the downward pressure of the other fingers that press down on the fingerboard (which produce different sounding musical notes). There can be a fine balance between the amount the thumb holds onto the neck of the violin and the amount you allow the thumb not to hold on and absorb the downward pressure from the other fingers on the fingerboard. It is also a good idea not to allow the left arm and hand to become rigid because of the amount of inactivity it has when compared to the movement of the right hand which holds the bow.