BALL – for hand shape, loosening the thumb, and vibrato
A tennis ball is a good generic size, but any that fits your hand will do.
HAND SHAPE – Collapsing the large knuckle on the hand is a common problem. Often the palm-side of the knuckle will grip the neck, creating a pinch between the thumb and the first knuckle. Holding the ball in the palm while playing scales will allow only the finger tips and thumb tip to contact the bass, forcing the knuckles out.
I mentioned only the concept to a student once and he came to his lesson the next week doing three octave scales while holding a tennis ball. Crazy! but it forced him to get into thumb position early, over the break at the saddle. In thumb position, he held the ball against his palm with his little finger, which kept it from pointing up or curling into the palm – another common problem.
THUMB – Obviously it is difficult to grip against the thumb while doing this, thus keeping the hand in front of the bass and forcing a better use of bass weight and balance.
VIBRATO – A flattened or collapsed hand and tight grip can make for a very tense vibrato. Also, our finger tips are tiny in proportion to the bass. Consider the proportion of a violinists finger tip to their violin. So a finger tip the size of a small ball would approximate the right proportion, plus its just plain fun trying to vibrate using the ball.