...in "Excellence Was Expected", the original designers and engineers for the 986 explained the spoiler in interviews with the author.
The designer wanted the rear end of the Boxster to drop like the rear end of the 550 Spyder, RS60, et al.
The aerodynamicist wanted the rear or the car higher for better aero--less drag, less lift at high speed.
The spoiler that pops up on the 986 was a solution that solved BOTH issues.
The pop up spoiler allowed the designer to design a rear end that sloped DOWN like the original racing Spyders, and still have the ability to raise up at speed and "spoil" the rear end lift, like a Gurney Flap.
Like others have posted above, it does not push down on the rear end, but it keeps the rear end from LIFTING at high speed.
For an autocross, no advantage.
