Blow Off Valves / Anti-Surge / Dump

Pictured is a HKS Blow-off valve courtesy of dreamcasteclipse.blogspot.com . Turbo charged engines operating at wide open throttle and high rpm require a large volume of air to flow between the turbo and the inlet of the engine. When the throttle is closed compressed air will flow to the throttle valve without an exit (i.e. the air has nowhere to go).
This causes a surge which can raise the pressure of the air to a level which can be destructive to the engine e.g. damage may occur to the throttle plate, induction pipes may burst. The surge will also decompress back across the turbo, as this is the only path with the air can take.
The reverse flow back across the turbo acts on the compressor wheel and causes the turbine shaft to reduce in speed quicker than it would naturally. When the throttle is opened again, the turbo will have to spin-up for longer to the required speed, as turbo speed is proportional to boost/volume flow. In order to prevent this from happening, a valve is fitted between the turbo and inlet which vents off the excess volume of air. These are known as a anti-surge, blow-off or dump valve. They are normally operated by engine vacuum or by electronic control.