Actually it nothing to do with a ships bulbous bow. The extension is actually an airfoil; a reverse wing. If you look at some of the earlier pics you can see the shape. I made it upside down because it fit the contour of the car better. The effect is the same either way.
Airfoils and Airflow [Ch. 3 of See How It Flies]
Section 3.8 on inverted flight.