It's not a true "arming/disarming" mechanism (i.e. girtbar). With the advent of the Super-80, Douglas developed a secondary handle on the tailcone door, that not only opened the door but pushes a "pin" up thru a small opening in the top of the door that (when the door is opened) will trigger the jettison of the tailcone, and inflation of the slide.
The "arming/disarming" of this door is basically a 12" by 18" flap (covered in the same fabric as the flight attendant jumpseat headrests) that covers this "secondary handle". When "disarmed", the flap is over the handle, so that it is not inadvertantly pulled by ground crew. When "armed", the flap, is flipped down over the primary door handle (which only opens the door to the tailcone and catwalk) so that the red secondary handle is exposed for easy access during evacuation. This is the same mechanism on all MD80, MD90, and 717 series aircraft. The next time you fly on one of these aircraft...notice the aft boarding door during boarding. You'll see the "flap" flipped up over the red handle, and only the silver handle exposed. After pushback, and until gate arrival, you'll see the "flap" flipped down and the red secondary handle (actually covered by a red "remove in case of emergency/evacuation" small plastic cover).
Hope this makes sense...once you look at it...it will.