I'm sure he knew the basic structure of the song before he started, but there's some obvious (and inordinately kickass) improv going on there.
OK, QUICK POLL: how many people picked up their respective instruments and started practicing after hearing this piece?
EDIT: OK, I've figured out the arpeggio accompaniment of the intro. It's played on a guitar, but it's ok for any instrument.
I'll post it if there are any takers. Well? WELL?