found this just now.. pretty neat:
"The regulations stated that the section of the bottom right behind the front suspension was completely flat, to prevent that the cars would have extreme ground effects bottoms. Porsche fitted the 956 with large venturis, starting right behind the mandatory flat section, making it the first the ground-effects Porsche. Because of its width, a flat engine is not ideally suited for ground-effects, but this was somewhat fixed by tilting the engine upwards. Combining the ground-effects bottom with a simple but effective body, the 956 generated over 3 times more downforce than the 917."
Porsche 956 - Ultimatecarpage.com - Images, Specifications and Information