Originally Posted by GregW / Oregon
It's to minimize the Caonda effect. It reduces drag and maybe slightly uplift. It's too small to produce downforce.
But, it should have a slight effect on mileage at speed and top speed. Henri Coanda was an Italian aerodynamacist that discovered this. See pages 137 - 138 of the 7/01 Road & Track. There's a diagram of a spoiler exactly like the M3s showing the effect.
See also: http://en.wikipedia.org/wiki/Coanda_Effect
Yep, there is no way that little plastic spoiler can create significant downforce; little surface area and not a structural component. It's mainly there to manage the flow. Turbulance/undesired pressure differences are not good things in general unless you are young and stupid like I was 15 years ago and got a kick out of riding a bicycle behind trucks at 120kph on highways.