A friend and I were discussing the methods of controlling a light piston cessna-esque aircraft in the event of a control surface failure. Most obvious is the ability to control the pitch effectively with power and trim, but what about roll/yaw?

Opening the door inflight, albeit slightly cracked, serves as a great method for turning the aircraft, but oddly enough it turns the aircraft in the opposite direction of the door!

Believing it would act similiarly to a rudder, I was very suprised to see this happen and haven't found anything that gives an explanation to why it does this. Anyone who has thought about this or tried it, is there a good aerodynamics explanation? I would love to incorporate this complex question into a commerical pilot quizzing session, but first I need to know the answer!