[Kenth]

Mike, if a lower pressure than atmospheric pressure of about 14.7 pounds per square inch is created by the fuel pump (or the moving action of the pistons in the engine under the closed throttle blades) opening the valve(s) to the lower pressure area, in the fuel pump or intake manifold, will cause the atmospheric pressure to to fill the void (push fuel or/and air) in that area.

From Doug Roe´s book "Rochester Carburetors":

If you are unfamiliar thinking about the density of the combustion charge during the intake phase, concepts such as mass flow, pressure differences and volumetric effect are likely to become confusing.
Here is a simpler description of the principle, worth pondering, which should make things a little clearer.

The engine is an air pump - it takes in air and compresses it.
A common assumption about how the air gets into the cylinder is that the piston sucks in the air - the piston creates a vacuum that pulls the air in.
However, it is effectively atmospheric pressure - about 14.7 psi or 1kg/cm2 - that pushes the air in, assuming its path is unobstructed.
At WOT, the atmospheric pressure helps to fill the void above the piston during the intake phase.
If you close the throttle valves to a small opening, a smaller amount of atmospheric pressure will pass the throttle, the result is a smaller amount of air to the cylinder and thus less power.
Consequently, a small venturi or throttle opening throttles the amount of air that atmospheric pressure can force into the cylinder.
Thus, it is the pumping of the pistons that creates a negative pressure in the inlet under the closed, or almost closed throttle valves, which the atmospheric pressure equalizes more the more the throttle valves are opened.