During the actuation of the flap, the drive frequency of the stepper motor is reduced as a function of the requirements for torque delivered by the motor in order to shift the flap. To that end, information is continuously gathered representing the requirements for torque delivered by the motor in order to shift the flap, the drive frequency is reduced, if appropriate, in response to a detected increase in the torque requirements and, when the drive frequency is below a predetermined maximum frequency, the drive frequency is increased, if appropriate, in response to a detected reduction in the torque requirements. In that way, the power of the actuators can be limited while coping with momentary peaks in torque requirements.