[Jared at Pelican Parts]

A blow off valve essentially controls the amount of boost pressure created from the turbo unit.

Assume that a turbo can produce 30lbs. of pressure. However you want to be able to control how much of that pressure goes into the engine. The Pop off valve bleeds off pressure until it reaches a certian amount and closes. It's basically a relief valve that works the same way as a fuel pressure regulator.