Why your confused on the flap is because of this of this question: Is it mechanical or vacuum secondary's??? Answer is both. The back barrels are opened mechanically, however, there is no accelerator pump back there. Without the flap, and without an accelerator pump, you'd have a massive bog when you floored the car. The flap "eases" the air and also the shape of it helps the siphoning effect, avoiding the bog. So, the flap is like an Quick Fuel adjustable vacuum secondary, while the back barrels are opened like a Holley Double Pumper.
