I did a little reading for you:
"The BOV has two chambers on either side of a diaphragm, one smaller and one large. When you boost, the chambers are pressurized, but the larger chamber has more contact area with the diaphragm so the diaphragm stays closed. However, when you let off the throttle, the large chamber is emptied, and the small chamber is not, because the VTV is inbetween the small chamber and the rest of the induction system. The VTV restricts flow in the reverse direction, so the smaller chamber doesn't instantly depressurize. This causes the diaphragm to be pushed open by the smaller chamber, opening the BPV.
If the vacuum line to the smaller chamber is disconnected, this chamber will always be at atmospheric pressure, so the BPV will open whenever you are in vacuum and will close whenever you are under boost. So, if you have been boosting and suddenly lift off the throttle, the BOV will open once you hit vacuum. However, if you do have the VTV, the BOV will open as soon as you lift off the throttle, and it will also have more pressure pushing it open, so it will all around respond better.
If the smaller chamber has a vacuum line to it with no VTV, than the BPV will never open."
MORE INFO:
"The stock BPV has a VTV (vacuum transfer valve) that looks like a check valve. The VTV has a check valve to allow boost into the lower chamber and a bleed hole to allow the BPV lower chamber to remain at a higher pressure than the top chamber for a short period of time. This allows the BPV to respond much more quickly than an aftermarket BOV. For the BPV to work correctly, you need the VTV. There is no adjustment on the BPV."
Hope this Helps,