Here you go mate

http://en.wikipedia.org/wiki/Ground_effect_in_cars

Most of it is based on Bernoulli's Principle on compressible flows

Which is also here

http://en.wikipedia.org/wiki/Bernoulli%27s_principle


Like you said - The basic idea is to create an area of low pressure underneath the car, so that the higher pressure above the car will apply a downward force