Re 'mechanism for the leap':

Judging by my son's reaction to success, and the fact that he's now great at challenging himself (except at the piano!), I'd say the 'mechanism' comes from his having realized two things:

1) 'Hey, once I get past the initial paralyzing fear, I can actually do x!'

(I think that's where the intense joy comes from, the beaming smile he gets on his face when he realizes he's mastered something he thought was impossible.)

and

2) 'I don't need anybody else to make me make that leap -- I can do it myself!'

It took a while, but he does seem to have realized a few years ago that he had it in him to push himself -- he didn't need Mom to do it anymore. The lingering doubts at the piano (*much* more mildly expressed than his old fears) are all that remains.