I agree that it might be a vortex generator of some kind which alters the flow over the car, or creates it's own small vortex which affects the larger one which would normally form.
I did some studies on pick-up truck roof cabs which intended to do something similar. However I am far away from doing something about it (spring project), and even farther away from proving how or why it may work.
Anything which lowers the air pressure at a critical point (aft) will result in sucking the air above it (at higher pressure) down. In the end, it will appear as though the overall flow is better.
