Probably more than you care to know but,,,
The primary side of the coil receives a DC pulse or "shot" in order to generate the secondary high voltage. The coil is a step up transformer having a primary winding (battery & ECM side) and a secondary winding (spark plug side).
Transformer action (voltage step up) can only take place when AC or pulsating DC is applied to the primary. Since we are not looking for a continuous spark across the gap of our spark plug, just one shot or pulse of DC is needed to create the secondary spark.
That was cool! Well written too!
Cheers!
On the late model bikes, the primary winding of the coil is "sinked" to produce the spark. All that means is one side of the primary is always hooked to 12V (the system relay provides that). The other side of this primary winding now needs Ground to produce current flow. The ECM provides this "ground shot" to the winding whenever a spark is needed. Since the late model bikes are single fire & each cylinder has it's own spark advance map, the coil is actually 2 separate coils in 1 package. The ECM provides a ground shot for "each" coil.
The interesting part of the whole process is that the spark across the spark plug is created when the ECM releases or removes the wire from ground not when it applies the wire to ground. You would think the Hi voltage was produced when you drove current through the primary but it is the opposite. The High voltage is produced in the secondary when the ECM removes the wire from ground.
Hope this helps..