As I understand it:
Four stroke (car) carburetors have an accelerator pump to shoot an extra gob of gas into the manifold when you suddenly floorboard it. Instead of that, outboard carbs have that "pocket" that fills with gas while at slow speeds. When you suddenly bury the throttle, gas is sucked out of that pocket to temporarily enrichen the mix, then after that, it is metered by the high speed needle.
Oversimplification, I guess, but that's all I know. Can't even guarantee that.