Some engines with two magnetos are configured with one impulse, and the other not. At the very slow starting RPM, the spark from a regular mag can be weak, so the impulse coupling gives it a little extra umph. The spark timing is also retarded during the start so that the spark hits the fuel/ail near top dead center, rather than 20 something degrees beforehand. If the spark ignites the fuel/air 20 something before TDC, it really sets you up for a kickback, which can be damaging to the starter system.
Some engines have two impulse mags, but if not, you'd rather that the non impulse mag not fire "early" and completely defeat the desirable retard effect of the impulse mag during a start. Once RPM picks up, this feature automatically disengages. Key start switches are (should be) wired to accommodate this.
It is unfortunate that topics of this secondary nature get glossed over, if mentioned at all, during ground school, and during flying. Instructors themselves probably have not come to understand these system details, and students don't know to ask. Thus the value in new pilots, and particularly owners, actually seeking out this knowledge about the aircraft they fly.
Coffee and sticky buns at the maintenance hangar can open doors of great wisdom to pilots....