Here is an explaination of how it works from the service manual. Keep in mind this is the "short" version of it....
When the ignition key switch is turned to the "RUN" position, battery voltage is applied to
both the ECM through the purple wire and the MPR through the red/purple wire. As the
ECM receives the "RUN" signal, it internally completes the ground circuit of the main relay
for a period of two seconds, energizing the ignition/injection systems for start‑up. As the
engine is cranked with the starter motor, the ECM receives the output signal from the crank
position sensor (CPS) and completes the ground circuit to the main relay for engine
With the main relay closed (completed circuit), DC current from the battery/charging system
is transferred through the 20 amp main relay fuse to the positive terminal of all ignition coil
primary windings. The negative terminals of the ignition coil primaries are connected to the
engine ground through the coils internal driver, which is triggered by the ECM. With the
coil drivers closed, an electric magnetic field is allowed to build‑up within the ignition coil.
As the flywheel rotates, the CPS senses the location of the 54 teeth on the flywheel and
supplies the trigger signal information to the ECM. The ECM utilizes the CPS information
and determines when to adjust the trigger signal to the coil driver of each ignition coil. The
coil driver then opens the coil primary ground circuit which allows its magnetic field to
rapidly collapse across the coil secondary winding which induces a high voltage charge
(approximately 10,000 volts) that fires the spark plug.