See the following MB post on the subject

http://www.autospeed.com/cms/article.html?&A=1102

Taken from above post:

The supercharger engages only as required; when it is not being driven, the intake air flows through its rotors (ie no external bypass is fitted). Supercharger operation is dictated by the Bosch ME 2.8.1 engine management system on the basis of engine speed and load. However, the supercharger, which is driven from the engine's serpentine belt, will not engage at engine speeds over 3000 rpm - if this occurred, the belt stresses would be too high. If required, the engine can develop 400Nm from 2300 rpm to 6100 rpm.