Just found out what that button...

[mole]

...on the steering wheel does You know, the "M Mode" one

Originally Posted by Ebay

BMW E46 M3 CSL Steering Wheel Trim With M mode Button
This is the top bit of trim off the M3 CSL steering Wheel
It has the M mode button which makes the standard M3 CSL more responsive on the gear change the acceleration and it hardens the suspension

http://cgi.ebay.co.uk/BMW-E46-M3-CSL...d=p3286.c0.m14