I’m running 4.1.7, I have a switch configured for RCx_Option=31 for “motor emergency stop”.
There is a video linked from Auxiliary Functions — Copter documentation which describes the motor emergency stop function, and the video shows that after triggering the emergency stop Ardupilot will disarm after 5 seconds unless the emergency stop switch is returned to its original position.
In my testing, the emergency stop switch worked fine to disable the motors, but it did not disarm after 5 seconds. Since when the emergency stop switch is turned back off the plane will spin up the motor immediately to match the throttle position, the fact that it does not auto-disarm is a safety concern.