A capacitor, exciter coil, magnet, and diode are the main components of a capacitor discharge ignition system. A permanent magnet is attached to the cam shaft near the "exciter coil". Due to the rotation of the cam shaft, these permanent magnets also rotate and thus a voltage is produced in the exciter coil when the magnetic force lines cut through it. When positive charge is applied to the top of the coil diode “d1” becomes forward biased and the capacitor becomes slightly charged. The stator is fully charged in three or four rotations of the permanent magnets depending on the arrangement of the circuit.

When the capacitor is fully charged, the "pulse rotor" starts to trigger. The pulse rotor rotates at one-third or one-fourth the speed of the magnet. So when the capacitor is fully charged it provides the trigger to discharge it at the right time.

At the time of pulse triggering, the current starts to pass through the thyristor (SCR) and the current stored in the capacitor passes through the primary winding.
  This current creates a spark between the two contacts of the spark plug.

The capacitor charge-discharge cycle is determined by the demand for spark in the engine cylinder. The circuit is then configured to be highly timed.