They can reverse the blade angle to accomodate inverted flight, usually by flipping a switch or setting the collective middle position at neutral and going bot ways, up and down for the approprioate switching, not exactly sure how they do it control wise, but to sum it up, the do reverse the blade angle, what is really crazy is how fast it had to keep flipping the blade angles to do the stationary loops and make it look seamless...the forces on those blades have to be tremendous.