By increasing airflow speed under the car by streamlining the underbody and reducing drag, dynamic pressure will increase, static pressure will decrease, the end result should be a net downwards 'lift', it should suck the car on to the road, hell im only going off what I have read, what race teams use and my knowledge of aerodynamics.
The rear diffuser needs to allow the airflow to accelerate and rejoin the air from above the car, if that one curves upwards and you have sufficient flow coming from the front the overall effect will be like that of an upside down aerofoil