A diffuser allows the air traveling underneath the car a place to expand and decelerate back to road speed as well as providing wake infill. As the air enters towards the front of the car it accelerates and reduces pressure. There is a second suction peak at the transition of the flat bottom and diffuser. The diffuser then eases this "high velocity" air back to normal velocity and helps fill in the base area behind the race car making the whole underbody a more efficient downforce producing device.
Understand that a true flat bottomed car (one without a diffuser) will produce downforce in and of itself when run in rake. Essentially the entire flat bottom becomes one large diffuser. It too has two suction peaks, one upon entrance, the second at the trailing edge of the flat undertray. A diffuser acts to enhance this underside suction, it acts like a pump, encouraging better flow under the car.
One thing to note is that the rear wing interacts with the diffuser "driving" it. The proximity of the low pressure side of the rear wing to the diffuser encourages better flow-through for the underbody.
video below describes the different types of diffusers and the
corresponding crash structure depending on the shape of the diffuser.
These models actually have a maximum height for the diffuser in order
to regulate the role of wind flow and down force in racing. The
featured video can be found on YouTube and is powered by Sky Rocket
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