http://youratt.com/attwilliams/en/blog
We have known since the end of 2007 that the cars for the 2009 season would have aerodynamics very different to those of 2008. Race car aerodynamics are the biggest contributor to their relative performance and are developed using thousands of experiments in large wind tunnels and very complex simulations run on high performance computers.
The new rules about our aerodynamics have also sparked controversy in the sport much of the aerodynamic performance of the cars comes from the diffuser, which is the part low down at the back of the car.