I believe the answer is: brain changes that greatly improve dexterity
As we are conditioning ourselves to do something with a certain part of our body, our brain would eventually improve and adjust itself to make us able to do that thing better, even on automatic function. The same thing happen to soccer youths who are continuously trained to use both of their leg to make a maneuver ever since they're little.
Piano players who do drills when they are children to improve dexterity in their left hands experience brain changes that greatly improve dexterity as a result.
