Abstract: A controlling motor for moving a stem along an axis has a hydraulic actuator with a cylinder in which a piston that is frictionally linked to the stem is slidingly and sealingly mounted, as well as a compression spring that is frictionally linked to the stem and acts against the actuator. The controlling motor has a trigger with an inflow area into which hydraulic fluid may be introduced, a pressure area that communicates with the inflow area through a diaphragm and is connected to the actuator, and a drainage area from which the hydraulic fluid may be evacuated without pressure, as well as a switching element that links the pressure area to the drainage area when a positive pressure differential occurs between the pressure area and the inflow area, and that otherwise shuts off the drainage area. The stem may be pushed out of the cylinder by the compression spring and may be drawn into the cylinder when hydraulic fluid is applied to the inflow area.