on the down stroke the opening of the crankshaft is also facing down from the carb thereby acting like a check valve.
As the piston travels down it also force the fuel mixture up through the side chamber.
it is imperative to get a good seal. some people use selicone to seal the back plate.
this will prevent the engine from running too lean (too much air) thereby running too hot, therby blowing out the glow plug.
As the exhuast travels out of the cylinder it will also acts as a seal. Back presure is needed.

the exhuast will also provide pressure to the fuel tank, thereby forcing fuel to the carb or needle valve.

once up in higher altitude, with the prop spining and the model moving forward. More air will be forced into
the carb. You must tune for a rich (more fuel) setting on the ground to compensate.

Weather also plays a role. if you tuned your engine prefect in the afternoon in warmer air (thiner).
it's not going to run the same the next morning (denser air).

the cylinder (sleeving) on nitro engine are also tappered. The diameter is less on top. this is also
how nitro 2 stroke engines create a seal. unlike the 4 strokes with rings on the piston.
This is why you need to break in the engine.