  Blow-Off Valves/Bypass Valves
Tech Note: A "blow-off valve" (BOV) and a "compressor bypass valve" (CBV) do the same thing, which is to release compressed air in the intake tract when the throttle closes, so as to prevent high pressure from smacking back into the compressor wheel of the turbo. If the valve vents the relief air to atmosphere, it's a blow-off valve. If it sends the air back to the intake, it's a compressor bypass valve.
The WRX and WRX STI -- and all Subaru turbo cars, for that matter -- come with a CBV from the factory, and they work pretty well. You MIGHT find yourself needing a stiffer valve if you are running a lot of boost. But most people buy aftermarket BOV's for the noise they make.
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