1. When the piston reaches the top of the cylinder, the spark plug ignites the air/fuel mixture, which explodes, driving the piston downward.
2. Expanding gases force the piston down and escape out the exhaust valve as it is exposed. The piston forces the next air/fuel charge below it up through transfer ports to the area opening above it.
3. When the piston is at the bottom of the stroke, the transfer port's top is exposed to let the air/fuel charge into the cylinder above the piston.
4. As the piston heads back up, it creates a vacuum beneath it that sucks the next air/fuel charge in through the one-way reed valve system. The piston also compresses the charge above it to be ready to explode when the spark plug ignites again.