On a Turbo you bleed it off to reduce boost and keep it closed to increase it with the Wastegate or Gates that are Controlled Electonically "EFI" or Manually with Boost Controller "Blow Thru Carb"
The bypass valve's job is to bleed off the boost in the intake track when the throttle plate is closed, this is so the Compressor does not get abused and reduces compressor surge. The bypass or pop off valve should stay closed on boost while the throttle is open until the throttle is let loose and the Throttle Plate closes. Swoooshhhhhhhh sound.
