When the door is armed, the cabin crew slides the little handle to AUTOMATIC. This means that when the door is physically opened, the slide will deploy. There is something called a girt bar at the base of the door. When the door is in AUTOMATIC, the girt bar attaches itself to the bottom of the door frame. When the door is opened, the girt bar tugs on the chute (which is normally inside the door - the big lump often seen on the lower part of an aircraft door) and the chute inflates.
Sometimes, the chute can fail to inflate and will just hang there, attached to the girt bar. In this case, there is a small handle attahced to a small rope. Pull this and the girt bar will manually inflate.
After landing, the doors are put back to manual. Just as an extra safety check, the crews look at the bottom of the door at a small coloured indicator. This shows whether the girt bar is still attached to the door frame. If it is, then there is a problem, as there is the potential for the slide to inflate when the door is opened. If this happens, emormous pressure would push against the person outside into the airbridge, or catering truck and can kill someone. In Cathay, doors are always opened from the outside, so there will always be someone in the way if the chute accidentally inflates. The reason the doors are opened from the outside, is that the slide is supposed to disarm, regardless of whether the door is armed or not, if the door is opened by someone on the outside.