On a dirt-bike in particular.
I'd have to watch a video of someone doing it on a dirt bike, but for a person on a trampoline or similar, the act of tucking your knees in lowers your moment of inertia and allows you to rotate rapidly through your center of mass. This is the same type of thing as a ice skater bringing their arms in.
That's the thing, on a dirtbike it's not possible to tuck your knees or anything.
Here's a video link http://www.youtube.com/watch?v=iCYSZtV6ZvI&feature=related
I notice that upon hitting the ramp, the nose of the bike is almost vertical, so a full rotation isn't done in the air.
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