Basicly with a "blow through" set-up the GM MAS is located after the BOV. It meters the air after the BOV and just before the throttle body. Because of this it doesn't get thrown off by the loss of air from the BOV when it open (e.g.) between shift. In all honesty, there isn't a lot of real benefit from it. It does make the car a bit less sensitive to some boost leaks (pre-throttle body elbow basicly) and might keep you from stalling out if you blow an IC pipe.
Some contend that a draw through might allow for slightly more precise metering although I've never seen any real evidence to that effect. Overall it's a matter of preference mainly. On the race car we run a "blow through" as the Tial BOV can not be recirculated.
Oh, and the shop phone has been operational again for a few days (after Verizon rewired half the building and then re-routed some things on their end). Although this morning it was coming up busy for hours until we put in yet another service call.

(It would ring at the outside service point but come through busy if connected through to the desk). Working yet again though as of about 2pm through when I checked late this evening. (fingers crossed).