[dizzyfingers]

I agree with your momentum theory but furthermore it seems to also have a strong synesthetic element. As I mature as a musician these synesthetic experiences become more vivid while in the midst of making music.

The best unscientific metaphor I can think of to describe it would be like casting the Patronus Charm from Harry Potter.

[agumonkey]

I don't know HP enough to get your analogy. When I'm in flow, music is like surfing a multi dimensional curved space. If my body isn't tuned enough and I'm not relaxed enough I lose "sight" of the surface and energy loss occurs, otherwise you just follow the valleys or hills as you see fit.