It's installed inline to the hose going from the wastegate actuator to the turbo. Just as the name implies, it bleeds off boost pressure in that line, thus making it take more boost pressure to move the wastegate rod to open the puck on the exhaust side of the turbo.
If the value is completely closed, the puck will move immediately when boost is generated causing terrible spoolup because all of the exhaus is going out the puck instead of turning the turbo turbine.
As the value is opened, more and more pressure is bled off keeping the puck closed and all of the exhaust gases spinning the turbo.
