Different answers for different cars.
Basically the idle valve goes to a limp home position when unpowered. Again this varies by system. The system used on 4 and 6 cylinder KE aquires an opening size that gives an amount of air equal to a slightly higher than normal idle speed with the A/C off and out of gear. That same amount is not very good when the A/C is engaged and the car placed in gear.
The early K-jet w/Lambda v8s had valves that went all the way open when unpowered.
The LH, HFM, and ME systems use throttle aactuators that are spring loaded to a position similar in concept to the KE cars (slightly higher when unpowered).
Bosch Master, ASE Master, L1
33 years MB technician