"The energy storage spring used to “snap" the shaft during cranking now keeps the magneto in proper step with the engine for running"
That's the bit I can't get my head round. The retardation is caused on startup by the pawls catching on the stop nut, winding up the spring which suddenly releases and gives a burst of energy. However once the engine is up and running the pawls no longer catch on the stop nuts due to centrifugal force and the spring is no longer wound.
I'm guessing that if the spring snaps then the pawls aren't held in place correctly and can catch on the stop nuts again. But I'd like an expert to confirm this !