Blow Off Valve
Blow Off Valve / Bypass valve releases the pressure from the turbo when the throttle plate is closed. The turbo is still spinning and still creating pressure when you let go off the gas pedal. The forced air will then hit the throttle plate and force the turbo to stop spinning abruptly, and cause damage.
When having a BOV mounted it reads vacuum from the manifold and it either opens or softens the valve. This lets the pressure escape through the BOV instead of hitting the throttle plate and cause the turbo to stop spinning.
On cars with Air Flow Meter the escaped air must go back into the intake before your turbo because the Air Flow Meter has accounted for it (By pass Valve). If not you will have a temporary rich condition which will upset your idle slightly.
On cars with MAP-sensor (Manifold Air Pressure) you can use Blow Off Valve which lets the escaped air out into the atmosphere.