A blow off valve or shift valve is a device that allows boost pressure to be relieved from the intercooler hoses when shifting and the throttle blade is closed. This has two benefits:
1. Turbo stays spooled up, there is no compressor slow down due to "stacking".
2. Turbo lives longer due to boost not "stacking" up on the compressor blade.
Stacking is when the pressure in the intercooler hose's is greater than the turbo output- that's the noise you hear when you shift with an open element air cleaner, and it's very bad for the turbocharger.