One of the most basic circuits found on every gasoline-powered vehicle is the ignition coil. This transformer takes low-voltage, high-amperage current and changes it into high-voltage current. It has two windings that are not physically connected. What connects them is magnetism. The primary creates an electromagnetic field when system voltage is applied. When the power is turned off, the magnetic field collapses. The energy from the primary is transformed by the secondary windings into high-voltage power that can jump the gap between the electrodes of the spark plug; this is called inductance.
It is a very simple and robust circuit. But, why does it give technicians problems when diagnosing an ignition related misfires? The answer is that some technicians use tests that might give inconclusive results or do damage to the coil or drivers inside a module.