When you are almost done with the hit, and you remove your finger from the 3rd hole, the rest of the smoke in the pipe, rushes into your mouth all at once. Since you are running out of Inhale Power, it helps you get the last of the smoke with greater ease.
in my mind its always just to kinda shoot the smoke further into your lungs by topping it off with the bit of air you get by releasing the shotgun
i've always thought of it as a way to clear out the smoke so it doesn't become stale just sitting in the pipe.....also as digitall said, to shoot the smoke into your lungs.