the reason for turnin of the alternator is because on new cars these days theres so many relays and crap
and the alternator and some thermo fans when spinin cause a tiny voltage to still run thru the system and its enough to keep relays on even when batt key is off
so by disconnecting alternator at same time as battery the engine will not keep running
this is a common prob on new cars
some battery switches these days have 2 big terminals for bat cable
then 2 or 4 small terminals for coil power so it cuts power to coils and the engine stops