The path to ground for the secondary is through the spark plug. That's what causes ignition.

 

That's not a HD secondary coil he has (had?) there. On a HD coil, which fires in waste spark, the secondary circuit is: Ground through one spark plug to coil to ground through the other spark plug. The cylinder not on the compression stroke acts as a low resistance and the one on the compression stroke acts as a high resistance.

With a single fire (or dual fire, whatever, not waste spark) coil where it is really 2 coils, like is pictured above, then the secondary circuit is (near as I can tell) the same as on a car: Ground through the coil body to coil to ground through the one spark plug. Unless they are doing something different.

 

Unless I'm missing something basic (which is certainly possible), the path to ground, even on a dual fire is through the plug to the head and so forth back to the battery. It just fires them both each time, knowing not which is on the compression stroke. The boom only happens on compression, but the plug not on the compression stroke sends the pulse to ground just the same. Single fire is just two coils firing at ONLY the correct time.

For terminology clarification, the primary coil circuit is what we would normally call the input, and the output.... to the plugs is the secondary.

At least that is my understanding, but I'm certainly open to correction.

 

When most of us refer to ground we mean a piece of metal on the coil that touches the bare chassis. Attached is a single fire coil and there is no chassis ground on it rather the body is all plastic. Current has to flow in one terminal and out another even if it's 1 milliamp at 20000 volts. If we assume current flows out the spark plug wire to make the spark, current must also flow in either the +12 terminal or the grounded minus terminal of the non-firing cylinder. The firing cylinder's minus terminal has just been ungrounded to create the spark so current can't flow through it.

This post was probably a mistake since to most people it will sound like nonsense but at least I included a pretty picture!

D-gyver included the perfect pic for testing a single fire coil though. Thanks for that.