A 2-stroke dirt bike engine works on two strokes or cycles of a piston. On one of the two strokes, the air-fuel mix is drawn into the combustion chamber via the transfer port from the crankcase. As the piston nears the top of the combustion chamber the spark plug ignites the air-fuel mix. This combustion creates high pressure, which forces the piston down on the second of the two strokes. As the piston rose during the first stroke, this reduced the air pressure in the crankcase. This reduced pressure draws in more air and fuel mix, in preparation for the next cycle or stroke. This process starts over again to start the first of the two strokes once more.