Turbosmart dump valves
Turbosmart items are top quality and often found on high performance builds and competition cars. They are not the cheapest, but have zero compromises.
A dump valve namely dumps excess pressure from the turbo pipes when the throttle body closes. Without a dump valve, the air does the same as a wawe in a bath tub: it bounces back. When air starts flowing in the wrong direction through the turbocharger, it can makes a nice chattering sound but in addition can seriously harm the turbocharger.
A bypass is the same as a dump with the expection that a dump frees the air into the atmosphere, and a bypass is connected to a hose which runs past the turbo back into the intake pipe.