It can open two ways.... if the pressure is more than 125 or so psi or when you let off the throttle. I'm sure it will only happen when you let off the throttle though.
You will need to go down to your local auto store and buy a vacuum t-fitting and some new hose to hook up your by-pass valve. The valve is open with vacuum (at idle) or as you let off the throttle. You should be able to hear it as you let off the throttle. You can have it vent to atomosphere or re-route the air to your blower inlet tubing (recommended).