Bump and some confirmed information that has been floating around for some time. Right from BMW training manuals:
Permanent all-wheel drive is cancelled to a large extent or completely in only three control situations:
• When negotiating extremely tight corners with little engine torque in order to allow speed equalization between the front and rear axle (e.g. parking)
• At speeds in excess of 180 km/h
• In extreme understeer driving situations
Also, interesting finding:
There is also a temperature sensor installed in the motor that signals the temperature to the transfer case control unit (VGSG). A temperature model is calculated in the VGSG for the purpose of protecting the motor from
overload. For this purpose, the maximum closing rate is reduced in various stages. If these measures are not sufficient to protect the motor from overload, the control is interrupted and the clutch completely opened so that only rear axle drive is now possible.