When you first learned to ride bike without training wheels, just riding around was enough, then all of a sudden you stood up on the pedals, and next thing you know you're jumping over your friends with rickety make shift ramps. The brain works in a similar way. Say you involve yourself in something, dedicate yourself, the brain will strengthen connections related to that activity. Involve yourself in something else, your brain executes a new program and repetition will strengthen connections related to that activity. Now say you take a break from that activity, you do something else for a while, those connections begin to atrophy because you don't use them, if they overlap with other activities, there will be something left there, but it will begin to tie into the new thing you do. Over time you create new web of connected brain activity to reflect what you do, and if you go back to what you used to do, you'll begin rebuilding those brain networks, except now they will be combined with the other stuff you were doing. You basically learn to ride a new bike, and next thing you know, you're jumping over your friends with professionally built ramps.