You can see how the rear wheel spins up, suddenly bereft of it’s previous load, thereby transferring angular momentum to the bike’s frame; this causes the m/c to rotate backward, raising the front wheel. Evel stands up to slow the m/c’s backward rotation by increasing the moment of inertia of the whole m/c-driver assembly. Had he not done that, he would have wiped out by landing with his front wheel pointed at the sky.
Very dangerous.
I think MORE frame lift would be generated with wheel TOUCHING the ground; not spinning freely in the air.
Perhaps some guys who paid more attention during physics class than I did, can shed more light on this.