Blow Off Valve

A Blow-Off Valve(BOV) is a component present in turbo engines with function to release a pressure. It is located right before the throttle body.
Actually it is a vacuum operated valve which releases air to the atmosphere or back into the intake.

It helps to protect our turbo system from 'compressor surges'.
These surges occur when throttle pedal is released while under turbo boost, normally when we changing gear.
When the throttle plate is closed, the high pressure air in the intake system is trapped by the throttle and a pressure wave is forced back into the compressor.
With no place to go, the compressed air will pushes back against the turbo, possibly damaging it.
The Blow Off Valve will react quickly to this situation and release the compressed air.

So, the turbocharger will keeps spinning even the throttle plate is fully closed.
Once the driver presses the throttle pedal once again, the turbocharger - which is still spinning - will keep on providing the optimal power .

Comparison between turbocharger and supercharger

The main difference between a 'turbocharger' and a 'supercharger' is its power supply. Something has to supply the power to run the air compressor.
In a supercharger, it use a belt that connects directly to the engine. It gets its power the same way that the water pump or alternator does.
While a turbocharger, gets its power from the exhaust stream. The exhaust runs through a turbine, which in turn spins the compressor

Basically, a turbocharger is more efficient because it is using the "wasted" energy in the exhaust stream for its power source. The bad thing is, a turbocharger causes some amount of back pressure in the exhaust system and tends to provide less boost until the engine is running at higher RPMs. Superchargers are easier to install but the price is more expensive.