as malcom said, this is the idea behind a diffuser (along with the coanda effect directing air upwards on a curved diffuser like on a lotus elise) this is what good ol' wikipedia says:
Rear
A
diffuser, in an automotive context, is usually a shaped section of the
car underbody which improves the car's
automotive aerodynamics properties by enhancing the transition between the high-velocity airflow underneath the car and the much slower freestream airflow of the ambient
atmosphere. It works by providing a space for the underbody airflow to decelerate and expand so that the boundary between the car's airflow and "external" airflow is less
turbulent, and it also provides a degree of "
wake infill" (the wake being a turbulent area of low pressure that is caused by the passage of the vehicle through the air; this can cause
pressure drag).
As the air enters towards the front of the car it
accelerates and reduces
pressure. There is a second suction peak at the transition of the flat bottom and diffuser. The diffuser then eases this "high velocity" air back to normal velocity and also helps fill in the area behind the car making the whole underbody a more efficient
downforce producing device by reducing
drag on the car and increasing downforce.
The aft part of a car underbody can be a
diffuser. It tries to connect the underbody to the back without producing
turbulences so that
Bernoulli's principle applies and the pressure increases while the velocity decreases. The side and the roof end in a sharp edge, so that their pressure does not increase. Because the pressure in the back tends to equilibrate, the pressure below the car is lower than on the side and the roof of the car.
so in effect having the rear higher creates a larger volume of 'space' underneath the car at the rear.
also, the airflow over the body is directed (slightly) upwards (along with the lip at the rear) creating even more downforce. basically more downforce for a tiny bit more drag and everybodys happy! downforce for everybody!
ps i'm not an expert on this, it just seems to make sense is all.