That force of that explosion forces the piston down.

爆炸产生的力迫使活塞向下。

And it's that pressure and the heat that actually ignites it, forcing the piston down.

正是这种压力和热量点燃了它，迫使活塞向下。

So going back to the four strokes of the motor, the piston is forced down, as the piston is forced down, it's forced down on a crank shaft.

说回到四冲程电机，活塞被向下推，当活塞被向下推时，它被压在曲轴上。