A blowoff valve (BOV) or dump valve is a pressure release system present in most turbocharged engines.
Its purpose is to prevent compressor surge, and reduce wear on the turbocharger and engine. Blowoff valves relieve the damaging effects of compressor "surge loading" by allowing the compressed air to vent to the atmosphere, making a distinct hissing sound, or recirculated into the intake upstream of the compressor inlet.
Other articles related to "blowoff valve":
... A blowoff valve is connected by a vacuum hose to the intake manifold after the throttle plate ... below atmospheric pressure and the resulting pressure differential operates the blowoff valve's piston ...