Compressor surge is when the boosted intake air has no where to go but back in the OPPOSITE direction of where your turbo is spinning. So your turbo goes from 30,000 rpm to 0 or negative in zero seconds flat. That WILL break your turbo. The blow off valve or by-pass valve ejects that compressed air so it doesn't go back into your turbo. The people who don't run BOV run BPV instead... both serve the same purpose of purging the charged air from the pipes. Coming from someone who actually owns a turbocharged car.