This Quicktime animation is a realistic simulation of the spark plug ignition sequence. Notice that the "Leading" (lower) plugs fire simultaneously, while the "Trailing" (top) plugs fire independently of each other. The extra spark of the leading plug late in the exhaust stroke is called a "waste spark". Using a waste spark simplifies the ignition system as only one coil and ignition signal are required for both leading plugs. The trailing plugs cannot utilize a waste spark because the timing would ignite the incoming air/fuel mixture in the next chamber.