A controllable butterfly valve in the exhaust pipe of a reciprocating engine equipped with an exhaust-driven turbocharger. When the waste gate is open, the exhaust gases leave the engine through the exhaust pipe. But when it is closed, the gases must pass through the turbine that drives the turbocharger compressor.

By controlling the amount the waste gate is open, the speed of the turbocharger can be controlled, and this speed determines the manifold pressure in the engine.