If you open the right door, that would deflect air out to the right... the opposite reaction would be the aircraft yawing left, and vice-versa. See Newton's 3rd law for further info.
If you were able to get the door open to 90 degrees, then maybe you would get the yawing effect caused by drag towards the open door. You'd have to be fast (and incredibly strong) to avoid the initial "wrong-way" yaw though. For smaller door opening angles, I think my first point is the reason. I think that's along the lines of what you meant by
air cushion??
I once tried steering a supercub with one arm out of the window, and one out of the door... didn't really work.
