On a waste spark coil, which has 2 terminals, one terminal is the "out" terminal and one is the "in" with a wire going from each terminal to a plug. The loop going from the "out" terminal goes through the first sparkplug wire to the first plug, has a spark, then grounds through the block (it gets tricky here) to get to the final ground the second spark plug has to spark and travel back through the second spark plug wire to the "in" side of the coil where it all started. This means for each time the coil fires it has to pass through 2 spark gaps to get back to ground.

In a piston setup this works because there are usualy 2 cylinders that go to TDC at the same time, so the coil fires every 360 degrees. This means that every time the coil fires, one of the sparks is igniting a pressurised fuel and air charge, and the other spark is just sparking while the piston is pushing the spent exhaust gasses. Hence the term Waste Spark.

So on a rotary, which needs to have the 2 plugs in each rotor, can't use the waste spark on a sigle rotor, because the plugs have to fire at different times. Having the loop go between rotors doesn't work well either since there isn't the best ground connection between the 2 rotor housings. So adding an auxilary ground for each waste spark coil is an efficient way to use the waste spark coils that come with a system like the Electromotive TEC EFI standalones.


I hope this helps