The dump valve is the black plastic saucepan shaped object that you will see connected between the intercooler and the air inlet hose to the turbocharger. You will also see a small hose leading from the top of the valve to the inlet plenum on the engine. Inside the valve are a rubber diaphragm, a steel spring, and a small insert to support the diaphragm where it seats against the dump valve body. The dump valve has basically two operating conditions; throttle open, and throttle closed. When the throttle is open, the pressure onto the top that comes via the small hose from the engin plenum chamber equals the boost pressure coming into the valve below the diaphragm. As the pressures are equal, the spring holds the diaphragm onto its seat, and prevents the pressure venting away. When the throttle is closed, the pressure coming into the top of the valve es significantly reduced, (often to a vacuum), and the pressure below the diaphragm lifts it away from its seat, allowing the excess pressure to vent away. As the throttle is reopened, the pressures again equalise, the spring pushes the diaphragm onto its seat, preventing further loss of boost pressure.