NIPPONDENSO Electronic Ignition System: Operation
As the signal rotor turns with the distributor shaft, its teeth (one for each engine cylinder) pass the magnetic pick-up coil assembly. See Fig 1.
As the air gap changes with the approach and passing of each tooth, the magnetic field varies. This creates a signal in the pick-up coil assembly. The ignitor senses this signal, and turns the ignition coil primary circuit on and off.
This causes the magnetic field in coil primary circuit to build and collapse, resulting in a voltage surge in the secondary, which fires the spark plugs.
Some models have EFI relays or other minor variations.