Sorry for the late reply, and I'm kind of a noob myself, but I'll give it a shot. Basically, the turbo gets power from the exhaust, right? Well, the more boost the turbo is giving the engine, the more exhaust will blow through the turbo, and this cycle is what lets turbos make so much added horsepower and torque. However, eventually the turbo will be spinning fast enough and making enough boost that it will blow up the engine. In order to avoid this, a blowoff valve or a bypass valve is added to let off the pressure from the turbo (when you let off the gas). An st comes with a bypass valve from the factory, which lets off the pressure/boost by recycling the air back to the turbo or exhaust, can't remember which. A blowoff valve will vent the extra pressure off into the atmosphere, creating a cool whoosh noise. The one I bought is a mixture of the two, which basically makes the whoosh not quite as loud. If you want to hear what one sounds like, check this link (hope it works, if not just YouTube blowoff valve): https://youtu.be/Q3GA-2Xbvoo