from m5board.com :
http://www.m5board.com/vbulletin/e60...ost434807.html
The mechanics of the P400 button aren't that hard to grasp. Remember that
you have an electronic throttle. All they have to do is NOT open the throttles
fully and/or de-optimize the camshaft positions. BMW already did this with
the '01/'02 M Roadster vs. the '01 M3. (315hp vs 333 by cam positions/etc)