
Continuity mistake: During the interrogation scene, when Batman beats up Joker, Joker's hands are holding Batman's forearms upside down (from underneath). When the frame changes his hands are now on top of Batman's forearms, then it cuts back and his hands are under again. (01:28:00 - 01:29:00)
Answer: The gun fires using compressed carbon dioxide as a propellant. The cable is wrapped in a coil around a motorized turbine within the gun. The pulley system in Batman's belt buckle allows him to reel himself in hands-free and still maintain equilibrium. There is a four-pronged grapple at the end of the cable. This is what is magnetized, creating an attraction between it and the monofilament cable. This ensures that the grapple will attach itself to the cable for a secure hold. This is why the magnet itself does not need to be especially strong to support any amount of weight. It also explains the relative ease with which it can be detached and its seemingly "magical" ability to wrap itself around various objects.
Phixius ★