When the engine is running, the ignition coil receives a signal from the engine control unit to fire at the correct time. The coil then rapidly builds up a magnetic field within the primary winding when the signal is received, and when the signal is cut off, the magnetic field collapses rapidly, inducing a high voltage in the secondary winding. This high voltage is then sent to the spark plug, where it jumps the electrode gap, igniting the air-fuel mixture in the combustion chamber.