A power transmission device for power transmission between an engine and a power output, comprising at least an input and an output, a hydrodynamic component with an impeller and a turbine wheel and a device for partially bypassing at least the hydrodynamic component with a servo unit featuring a piston element that can be pressurized with hydraulic medium and able to influence the circulation rate difference of flow media on both sides of the piston element, wherein the influence comprises rotary synchronism with the piston element of the servo unit, a further rotatable piston element disposed in axial direction with a spacing from the piston element of the servo unit.