How Does a Blow Off Valve Function?
Proper blow off valve function is essential to receiving optimal performance from your car’s turbocharged engine. The blow off valve in your car is designed to route compressed air away from the throttle body when the throttle is suddenly lifted in a turbocharged engine. This allows the turbocharger to continue spinning at the faster speed to decrease lag time in throttle response when you get back into the throttle. When you lift your foot off the accelerator pedal, the throttle body closes and vacuum in the engine goes up. The blow off valve has a vacuum input on it that opens the blow off valve routing compressed air either to the atmosphere, or back into the intake stream somewhere before the compressor.
For more info, see Do you Need a Turbo Blow Off Valve?