It's called Bernoulli's Law. The speed of air increases when air flows through a venturi (a thing that is kind of shaped like a hourglass). Air pressure decreases as the air speed increases. Fuel is held in a chamber vented to atmosphere. Air passing through the venturi when the engine is running creats a low-pressure area while there is higher atmospheric in the float bowl. That's how fuel gets sucked into the carb. The throttle controls the amount of air flow with the throttle plate. So, if you open the throttle plate under acceleration your vacuum "bleeds" off. Let off the throttle and the venturi gets more constricted and low atmospheric pressure or vacuum is there again. Got it?
