Rexroth DBD pressure relief valves are the direct operated pressure relief valves, the oil is pressurized at the bottom of throttle piston against the spring mounted in the housing. The seat is closed when the hydraulic pressure force less than the spring force, the valve cannot be adjusted at the moment. When the pressure is increasing larger than the spring force, the seat is pushed upwards allowing the oil into the reservoir and keeping the pressure on inlet port at a stable value.