A diffuser, in an automotive context, is a shaped section of the car rear 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.

What is the purpose of a rear diffuser on a car?

The diffuser acts as an expansion chamber to manage the air as it exits from underneath the car and reintegrates it with higher-pressure ambient air. Smoothing this transition reduces turbulence and drag in the car's wake and improves airflow under the car.

Does rear diffuser do anything?

The main function of a car rear diffuser effectively increases the vehicle's downforce and increases the car's grip on the road surface and reduce the aerodynamic drag. ... With low air pressure under the car and high-pressure above, increased downforce is created and the aerodynamic drag is reduced.