In a two- stroke engine the vacuum created by the piston rising sucks a mixture of air/ fuel/ two- stroke oil into the crankcase under the piston via a valve (generally rotary or reed). While this is happening, the piston is compressing the previous intake mixture of air/ fuel/two-stroke oil, which is then ignited by the sparkplug, forcing the piston back down again.
As it travels downwards, the piston’s motion increases the pressure on the mixture in the crankcase, causing it to travel via an intake port to the combustion chamber above the piston. This new fuel helps push out the burnt gases via the exhaust port before the piston rises again, closing the exhaust port as it compresses the fresh air/fuel/two-stroke oil mixture and sucks in another load beneath it, thus starting the whole sequence again.
It’s called a two- stroke because of the piston’s two movements (strokes) up and down the cylinder between each combustion sequence. A four-stroke separates the same functions into four distinct phases (suck, squeeze, bang, blow) – but that’s another story.