if the alarm goes off initially and is not shut off it will go off again in five minutes hence the back up.
On my DroidX, when the Alarm executes and rings for 5 minutes, it will then go into the snooze mode. It will snooze for 5 minutes, and then execute again.
In Alarm settings, you can change the "snooze duration" but cannot change the ring duration. Ring duration seems always to be 5 minutes.
Selecting or not selecting "BackUp Alarm" option does not change any of that and I do not hear any playing of a "loud beep to get my attention if the alarm sounds for more than 5 minutes" at any time.
I still do not understand the purpose of that "BackUp Alarm" option.
Maybe someone has experience with this "BackUp Alarm" option and can provide some insight?