Functional description, see BMW Technology:

TIS -> Document -> SI Technology -> SBT Number (65 04 04 099)

Note:

All work on the airbag must only be conducted with the battery disconnected!Connect and disconnect control unit, sensors and inflator assemblies (gas generators) only with the battery disconnected!The MRS control unit contains vehicle-specific data and must therefore be encoded before placing into operation!Removal and installation in other vehicles is prohibited!