Atmospheric pressure will diffuse from high pressure to low pressure. When the piston of a two-stroke engine is at top dead center (or the piston of a four-stroke engine is at bottom dead center), a low pressure develops under the piston in the crankcase (above the piston of a four-stroke engine). At the same time, this low pressure will also cause low pressure in the carburetor. Because the pressure outside the engine and carburetor is higher, air will rush through the carburetor and into the engine until the pressure equalizes. The air flowing through the carburetor will carry the fuel, which will then mix with the air.
Inside the carburetor is a throat. The throat is the constricted section inside the carburetor that forces air to accelerate through it. A sudden narrowing of the river can be used to illustrate what happens inside the carburetor. River water will speed up as it approaches a narrowing bank, and it will be faster if the bank is continuously narrowing. If the same thing happens inside the carburetor, the accelerated flow of air will cause the air pressure inside the carburetor to drop.