The MDrive will engage which ever setting you choose on the MDrive menu.
The S1, S2 etc refer to the speed of the transmission, how fast it shifts. The higher the number the faster the shift. Most folks have their MDrive set to S5 or S6. S6 is only available with the MDrive button on.
In the MDrive menu you choose a setting for each variable, S5 or S6 for the transmission, Power On, etc. When you have those variables set , those become the settings when you press the M button. Hit the M button again and the car reverts back to what ever settings you were in, S4, D4 etc.
Current: 2011 Space Grey E90 M3, DCT, Comp. Pkg., Conv. Pkg., everything but Cold Weather Pkg!
Gone: 2008 550, Space Grey, Sports Package, Sport Auto Transmission.
Sold: 2004 545i (U.S.), Steptronic, Silver Gray.