Not really, that's just to relieve the high pressure building up behind the bumper (in an effort to reduce aero drag). I'll actually have to disagree with Kamin on his definition of a diffuser, as an air damn and an airfoil diffuser are two very different things IMHO. An air dam is just as he described it, a "dam" that attempts to prevent airflow from reaching certain points. These are often used in conjunction with front splitters to direct air up and over, or around the sides of the car rather than under it (which is usually a very aerodynamically dirty area). A diffuser on the other hand is an airfoil (any device designed specicially to alter airflow) who's primary purpose is to help evacuate air from under the car in a controlled manner by creating an area of strong low pressure to the aft of the vehicle. So an air dam attempts to prevent air getting underneath the car, and a diffuser attempts to evacuate what air does get under effectively (while also influencing the Center of Pressure, the aerodynamic equivalent of Center of Gravity). A picture of the *** end of a diffuser section (notice the smoothly upward arcing aluminum airfoils)...