...it's a Gurney Flap.
What it does, is build a high pressure pocket of air as the airflow gets "dammed up" in front of the Gurney Flap.
The faster you go, the more high pressure air builds up in front of the spoiler, and the area of high pressure builds further forward until the entire top of the trunk to the back window is one, huge, high pressure air zone that pushes the entire back of the car down onto the pavement to reduce rear end lift, thus allowing you to maintain control at 160 mph.
The 986 Boxster is engineered so that there is more downforce on the rear, than the front as speed builds, thus promoting understeer in high speed corners for safety.
The Gurney Flap also reduces drag. The 986 has a .31 CD, which is pretty darn good for a convertible.
So, it's a very well engineered part, works great, and should be loved and admired and not messed with.
