I'm gonna try another way to explain this.
There is 14.7 psi of atmospheric air pressure at sealevel, right? Okay. This air pressure is forcing itself on the throttle plates in the carburetor when they're closed.
The engine's basically an air pump. So when it's running at idle, the idle circuit allows just enough air and fuel in to keep the engine running. But it's pumped most of the air out of the intake manifold AFTER the throttle plates. So you have vacuum. But it's vacuum as compared to the ambient air pressure. Your cam grind has something to do with how much vacuum you pull because of overlap. Torquey cams like those in truck engines are what they call "mild", which will have a higher vacuum than a drag race cam with lots of overlap.
But anyway, Once you open the throttle and the plates in the carburetor open up, all that ambient air pressure around you wants to force itself into the manifold and engine because nature hates a vacuum. Your engine gets more fuel with that extra oxygen because bournelli's theorem draws the fuel into the airstream. The engine responds by increasing in rpm, and because the Earth's atmosphere has just forced itself into the intake manifold, you will experience a drop in pressure simultaneously.
Does that make more sense?
