Those things gave me a fit back in “The Day” because I really didn’t understand them. But with what I know now, they aren’t all that complicated. It helps to first understand how it works as applied to horizontally opposed twins (picture below). When voltage in the secondary winding is high enough, the electrictal flow is as follows. It flows from one end of the coil, to a spark plug, jumps the gap to ground, THROUGH THE GROUND back to other spark plug, jumps the gap, and returns to the other end of the coil, completing the circuit. Thus, both plugs fire at the same time.
With alternate firing twins, one end of the coil secondary is grounded by the carbon brush, Now the flow is from the ungrounded end of the coil, to the other spark plug, jumps the gap to ground, and back to the end of the coil that is grounded by the brush, completing the circuit. When it is time for the other cylinder to fire, everything is reversed and the other plug fires.
