One thing about having the spill plate being triangular in shape similar to the McLaren that Neil provided is that this shape when it become side ways to the air flow, as in the. rear end getting loose, it will generate a huge amount of aero force that opposes the spin force of the car. The "delta" shape is extremely efficient and works by generating a very strong vortex on the opposite side of the spill plate that generates the differential pressure on the plate that opposes the spin forces.
Rex