Basically, in order not to destroy the motor if the window becomes stuck, the main gear in the centre of the motor is surrounded by several plastic pieces [pucks] which are designed to break should that (stuck window) occur. Of course, this requires the [Careful] removal of the window and motor, opening the motor and cleaning/repacking with new grease and replacing the pucks and testing. If the motor is damaged beyond repair a replacement motor may be required: watch this video as it shows the inside of the motor and when the gear (or pucks) has/have failed.