Timing of the spark is set upon positioning of the coil of wire, and the flywheel is not allowed to change its angular position on the crankshaft through the use of a key. At Top Dead Center (TDC) of the compression stoke, the magnet charges the coil, and a few degrees rotation of the flywheel later, as the magnet moves past the coil, the spark plug is fired just as the piston starts its power stroke. At the bottom of the of the power stroke, the magnet is on the other side of the flywheel. Piston starts to come back up (Exhaust Stroke), and at Top Dead Center again, the magnet passes by the coil and makes the spark plug fire again. But, because the piston is at the end of the exhaust stroke, and the beginning of the intake stroke, the spark is wasted since there is not anything in the cylinder to burn. As piston hits the bottom of the intake stroke, the magnet is once again on the other side of the rotating flywheel. When piston comes up during the compression stroke, the magnet energizes the coil of wire again, spark plug fires, and now ignites the compressed fuel/air mixture. This engine is known as a 4 stroke (aka cycle), the spark plug fires on every revolution of the engine, but you only get 1 power stroke (bang!) every 2 revolutions.