Simply put, when an engine is running boost from a blower or a turbo, which means the blower or turbo is forcing more air into the intake than the engine would naturally pull in, as you let off the accelerator that built up air pressure needs to escape. A blow off valve or a bypass valve senses the change in vacuum caused by letting off the accelerator and a diaphram opens the valve to release the pressure that would otherwise build up in the tubing or intercooler and over pressurize them. For example, if I didn't have the bypass valve on my Procharger all of that built up air pressure from the blower needs to go somewhere when I let off the gas and the throttle body closes. The valve then opens and releases the pressure into the atmosphere rather than causing my tubing to swell up and cause possible damage to it and the intercooler.