Video narration
How can two strokes do the work of four? A hand pump moves air as you push and pull. A piston moves gases through an engine in a similar way. As the piston rises, it squeezes the fuel-air mixture in the cylinder while drawing fresh mixture into the crankcase, the space beneath it. Near the top, a spark ignites the compressed mixture. Combustion drives the piston down. Here's the turn: this stroke makes power and compresses fresh mixture in the crankcase for the next cycle. Near the bottom, the exhaust port opens first. Then a transfer passage opens, letting compressed fresh mixture flow into the cylinder as exhaust escapes. Now it's ready to repeat. A typical two-stroke engine fires once per crankshaft turn: the upstroke compresses the charge; the downstroke makes power while moving fresh mixture in and exhaust out.