An air bypass valve or a blow off valve, BOV, can be installed into a car’s turbocharger system as a form of anti-lag system. Without some sort of anti-lag system, when you suddenly lift off the accelerator, a wave of compressed air will hit the closed throttle and bounce back into the air intake system, impinging against the compressor turbine and slowing it. It then takes time for the compressor to come back up to speed. This time is called turbo lag. A BOV operates off engine vacuum and vents the air charge either to the atmosphere or into the intake stream before the compressor. An air bypass valve causes the throttle to crack open ten to 15 degrees, allowing some of the air charge to enter the engine, or bypasses the air charge directly into the exhaust manifold.
For more info, see How to Choose Turbo Performance Parts