The blades change from a positive pitch to a negative pitch. When you fly up-right the blades have a positive pitch, once the model is flipped inverted you must bring the blades down to a negative pitch or angle of attack. The model stays in the air by accelerating air down towards the earth...no matter what attitude the model is in air has to be accelerated down towards the ground to stay at a given altitude, so when you are inverted the pitch of the blades simply changes so the air can be shot downwards.

Colin