A blow-off valve ( also called a compressor bypass valve)

Video Summary
<p>generally a piston type, which is placed between the turbo compressor and the throttle to bypass the pressurised air on a closed throttle, either plumbing it back into the turbo inlet for silent operation, or to the atmosphere. If it had not diverted the air, the air would have caused the turbo to start spinning backwards as the air was sucked back into it.</p>