I can't give you specifics of how the compressor works internally, but here is my understanding:
The compressor runs all the time, and in order to keep the evaporator at optimum temperature, the compressor's "compressing" is variable. In older MB systems the compressor cycles on and off, but the newer models don't. This means that the compressor is running constantly and this is better for compressor.
Steve Brotherton knows more about these systems than anyone I know, and might be able to provide specific details of how the systems work.
1998 C230 "Black Betty"