The "shelf on the orecas back is where the air-flow from the venturi-tunnels exit. Found this on some site:
Glossary
18. Venturi: A narrow tunnel under the side pod, shaped like an inverted wing. As air enters and is forced through the narrow center,its speed increases, creating a low pressure area between the bottom of the car and the track. This creates a suction effect, which holds the car to the track.
19. Venturi Effect: Fluid speed increases when the fluid is forced through a narrow or restricted area. The increased speed results in a reduction in pressure. The underbody venturi is shaped to create a low pressure area between the road and chassis which creates downforce.
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