The crankshaft is responsible for converting the explosive power generated from the combustion chamber, which is transferred through the piston and connecting rods, which in turn are connected to the crankshaft. It in turn controls the camshafts via the timing belts or chains and they control the valves and so the cycle continues. The crankshaft can be viewed as the backbone of the engine. The crankshaft is connected to a flywheel, which helps smooth out torque characteristics and this is in turn connected to the clutch.
Essentially as the piston is moving up and down during the different stages of the OTTO cycle, the downward pushing forces are converted into rotational motion which can drive the wheels via the drive-line, through the flywheel.
It is important when other tuning upgrades are applied to the engine increasing BHP and torque levels above standard specification, that careful consideration is taken to the crankshaft. As the additional loads and stresses can cause the crankshaft to become strained and in extreme cases break and destroy your engine.