In systems of this type, the distributor is eliminated and each spark plug is fired by a high voltage impulse from an ignition coil shared with a companion cylinder, or by an individual coil specifically for that cylinder.
This is referred to as direct fire ignition, or waste spark.
In a waste spark system for a six cylinder engine, three ignition coils, each with their own primary and secondary windings, are combined to form one coil pack.
Each ignition coil serves two cylinders, with each end of the secondary winding attached by a high tension lead to a spark plug. These two plugs are on companion cylinders. That is, cylinders where the pistons reach top dead center at the same time. The cylinder on compression stroke is said to be the "event cylinder" and the cylinder on the exhaust stroke is the "waste cylinder".
When the high tension voltage is induced in the secondary winding, the secondary circuit is completed by current flowing through the high tension lead to the center electrode on one spark plug; bridging the gap and creating a spark in that cylinder; traveling through the cylinder block; bridging the spark plug gap in the companion cylinder; and flowing back through that high tension lead, to its starting point.
The cylinder on the compression stroke with its charge of fuel and air, is "fired" by its spark, driving the piston down on the power stroke, while the spark at the plug of the cylinder on exhaust, simply serves to complete the circuit, and is "wasted".
When the crankshaft rotates through one revolution, the roles are reversed and the waste cylinder becomes the event cylinder. Firing of both plugs again takes place, and this cylinder now drives its piston down on the power stroke.
This same process occurs on each pair of companion cylinders as they approach the top dead center position. The primary circuit in each coil must therefore trigger at the correct time in each crankshaft revolution.