As I understand it with regard the topside of the car the air flowing over the car gives both down force and uplift.
When the air flows over the bonnet it acts as a lift and upon arriving at the front screen will then cause both drag and down force as it interrupts and causes higher pressure.
When past this and on the roof of the car a huge uplift is created and further more when travelling over the rear screen/boot areas.
Thus when travailing at higher speeds the back of the car goes light due to the air literally lifting the car from its footings.
The underside im yet to understand with regard to splitters channelling and rear diffusers.