[NZGTRA17]

Another similar diffuser article:

A rear diffuser helps drive the under-car flow by exposing it to the turbulent low-pressure wake region behind the car, using this low pressure to suck the flow out. In addition, the diffuser slows the air emerging from the underbody region by expanding it through a larger-area opening. They are effective in generating large amounts of downforce by increasing air speed underneath, thereby reducing pressure. Since this low-pressure region acts on a large surface area, plenty of downforce can be generated. Even if pressure below the diffuser is only half a psi lower than outside, over a 3x6-foot area, that equates to over 1000 pounds of downforce.
Vertical fences are installed within the diffuser channel to ensure that flow remains attached to the diffuser. Since the diffuser ceiling slopes upwards, airflow there is slowing down, resulting in increased pressure. Aerodynamicists call this type of region an adverse or unfavorable pressure gradient, since maintaining attached flow almost always requires that the flow speed increase throughout the region where it moves over a surface. Flow separation - and the resultant loss of flow velocity - would reduce downforce significantly if nothing were done to prevent it. The fences act as vortex generators to assist in energizing the flow through the diffuser, which help maintains attached flow and allows the air to fill in the wake.

Kel.