The WonderThumb is an aid to attaining a relaxed and functional left hand position for violin.
Remove the worry Reduce the tension
The Wonderthumb slides on to the thumb and transfers the support of the neck of the violin directly down to the base of the hand. This eliminates the possibility of the violin slipping down. No need to squeeze your hand to prevent the violin from sliding down the thumb. The hand is now allowed to RELAX. By keeping the neck of the instrument in correct position, the hand is then free to just finger the notes. The thumb mounting allows for independent movement of the fingers, maximizing dexterity.