This is simple people, if the vault door(aft cabin door) is armed, when it is opened, the tailcone drops and slide inflates.
If it is not armed, you can open the door all day and nothing happens. Let's say the F/A decided for some weird reason to disarm the door in an emergency, open the vault door, and then pull the secondary handle in the tail compartment, the SAME thing happens. The tail cone drops, and the slide inflates. If the tailcone is on the ground, the slide is as well, or it wasn't rigged right.