MAS is the combination relay that handles the heavy current users of the engine: fuel pump, A/C clutch, air pump clutch maybe others.
The two positions are connected and disconnected. The thing is so big that I guess MB decided there needed to be a lock to keep the thing engaged. If you turn the knob, the relay will rise from its position and disengage itself. It screws itself back in also (if you have everything lined up).
Bosch Master, ASE Master, L1
33 years MB technician