The starter motor is responsible for turning the engine over during ignition and allowing everything else to happen. When you turn the ignition on, the starter motor engages and turns the engine over allowing it to suck in air. On the engine, a flywheel, with a ring gear attached around the edge, is fitted to the end of the crankshaft. On the starter, the pinion is designed to fit into the grooves of the ring gear. When you turn the ignition switch, the electromagnet inside the body engages & pushes out a rod to which the pinion is attached. The pinion meets the flywheel and the starter motor turns. This spins the engine over, sucking in air (as well as fuel). As the engine turns over, the starter motor disengages, and the electromagnet stops.