A diffuser, in an automotive context, is a shaped section of the car underbody which improves the car's aerodynamic properties by enhancing the transition between the high-velocity airflow underneath the car and the much slower freestream airflow of the ambient atmosphere. It works by providing a space for the underbody airflow to decelerate and expand (in area, density remains constant at the speeds that cars travel) so that it does not cause excessive flow separation and drag, by providing a degree of "wake infill". The diffuser itself accelerates the flow in front of it, which helps generate downforce. When a diffuser is used, the air flows into the underbody from the front and sides of the car, accelerates and reduces pressure. There is a suction peak at the transition of the flat bottom and diffuser. The diffuser then eases this high velocity air back to normal velocity and also helps fill in the area behind the car making the whole underbody a more efficient downforce producing device by reducing drag on the car and increasing downforce. The diffuser also imparts upward momentum to the air which further increases downforce.
I have drug going up gravel driveways before and have not hit my front fascia.
Sonic's MK3 Crew #9 Chaos!