Operation of BOV

A BOV or a blow off valve is connected to the intake manifold by a vacuum hose. When the throttle closes, the relative pressure of the intake manifold falls below the zero bar.  It is this difference in the low and high pressure or the differential pressure, as it is known, that operates the piston of the blow off valve. Then the excess pressure of the turbocharger is let out into the atmosphere or redirected to the intake upstream of the inlet of the compressor.

Explanation of the terms used

Bar: Bar is a pressure unit of 100 kilopascals and equals Earth's atmospheric pressure at sea level. 

Intake manifold: The key feature of the intake manifold is either the equal distribution of air for direct injection engine, or directing the combustion mixture to every port of the intake manifold in the cylinder folds. The distribution or allocation function is very significant in enhancing or optimizing the performance and adeptness of the combustion engine. The intake manifold can also be used as a carburetor mount, fuel injectors, throttle body, and other engine components. A partial vacuum is said to be present in the intake manifold because of the hindrance created by the throttle valve and the downward movement of the pistons.  

This manifold or partial vacuum can also be used for driving auxiliary systems. Apart from that, it can also be used to suck the exhaust gases and unburned fuel that enters the crankcase of an engine. 

The intake manifold was mainly manufactured from cast iron or aluminum, but nowadays composite plastic materials are also being used.In the picture you see the BOV and the intake manifold.