As for roof flaps, it is pressure differential that makes them work. When going forward the air flowing over the roof makes the flap try to raise( moving air = low pressure), while also pushing them down (flowing air pushing the surface down). When you go the WRONG direction the flowing air catches the lip that was raised by the low pressure,raising the flap, thereby GREATLY reducing the lift of the car. kinda like the spoilers on a performance sailplane.