Thats not how the back doors work. I'll explain it using the Novas:
When you turn the door handle to rear on the local buses, the back doors will unlock (green light comes on) and will stay unlocked until someone touches the tape/bar, in which the doors will open and stay open. When the operator turns the handle to close, the doors will start to close. If the sensor picks up something in the doorway, the doors will automatulically reopen and reclose as many times as it needs until they're closed and locked.
On SBS buses/buses set in SBS mode, the doors automatically unlock and open once the driver turns the handle to rear. As with the local buses,once the handle gets turned to close, the doors will close and recycle if the sensor catches something in the door area.