My girlfriend who is a doctor has always been fascinated by the flexibility of my hands.
That thumb thing is not particularly good, moing your fingertip without moving the finger isn't good either.In fact, i'd say these are signs of inflexibility. If you press a key with your finger as in picture A the second joint will be weakened.Look at this mans hand, the middle finger:This is what happens (in the extreme)
B:
This is generally called "hitchhikers thumb" and is heritary. I don't have it, but know people who do. I would not have thought that this would aid pianism, unless you're playing a thumb-contorting concert!