Modern cars makers try to keep the bottom of their cars as flat as possible to minimize drag and maximize downforce. In the case of the drag, obviously a smooth surface will create less drag than an irregular one (such as the bottom of your car). For downforce, its used like an upside down airplane wing. Airplane wings create lift through their shape because the shape of the wing causes air to move faster above it than below it. That creates low pressure and essentially sucks the plane's wing upwards. Same thing applies to undercar surfaces and rear diffussers, except instead of creating lift ABOVE the car, you create the low pressure underneath the car, so that it sucks the car towards the ground.
A rear diffuser creates the same effect underneath the rear of the car. It creates negative lift and cleans up the air underneath the rear, effectively adding stability and downforce.
Here's s pic of modern day supercars to give you a better idea...
Enzo:
GT-R
In the 70's they tried installing big fans that would suck the air out from underneath the car, effectively vacuuming it to the track. That got outlawed quick... heres a quote:
"Lotus, in 1978, developed a fully shaped underbody to its Formula 1 car that, along with sliding side skirts, created a low pressure area under the car allowing cornering speeds to soar. The side skirts would wear and often break and the sudden loss of vacuum would result in catastrophic accidents. This technique of ground effects was also banned."
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Makes for a fun read.
http://jimsgarage.wordpress.com/2007...round-effects/