This Figure shows the hydraulic power steering assembly The torque sensor in the steering system in following way As the steering wheel rotates, so does the steering column, as well as the upper end of the torsion bar. Since the torsion bar is relatively thin and flexible,[3] and the bottom end usually resists being rotated, the bar will twist by an amount proportional to the applied torque. The difference in position between the opposite ends of the torsion bar controls a valve. The valve allows fluid to flow to the cylinder which provides steering assistance; the greater the "twist" of the torsion bar, the greater the force. Since the hydraulic pumps are positive-displacement type, the flow rate they deliver is directly proportional to the speed of the engine. This means that at high engine speeds the steering would naturally operate faster than at low engine speeds. Because this would be undesirable, a restricting orifice and flow-control valve direct some of the pump's output back to the hydraulic reservoir at high engine speeds. A pressure relief valve prevents a dangerous build-up of pressure when the hydraulic cylinder's piston reaches the end of its stroke. Some modern systems also include an electronic control valve to reduce the hydraulic supply pressure as the vehicle's speed increases; this is variable-assist power steering