thye BOV gets hooked to the intake manifold, when the throttle body closes and the BOV sees vacuum, the BOV opens to release pressure in the intercooler piping
the spring in the BOV should be strong enough that it only opens when theres vacuum in the manifold AND pressure in the intercooler piping
since your gauge should also be hooked to the intake manifold, it will read manifold vacuum when ever the BOV opens