To those who don't know:
Diffusers are underneath the car, they are designed to add downforce from under the car, which in turn keeps the car center from lifting into the air. The rear wing from R/C cars only provides downforce to the rear tires, and the design of the lexan bodys front end provides downforce in the front tires. Adding a rear diffuser will give the car more downforce in the rear.
If you can successfully create a chassis with a diffuser that has enough downforce in the rear, it will eliminate the need for a rear wing, thus reduces drag from the body and gives you more speed.
That is why it is outlawed.
edit: wrong car for this type of physics....touring cars don't have this kind of diffuser.