Continuity mistake: When Batman slams Joker's head against the glass after he says he took Rachel too, there are no pieces of glass on the wall shelf, but later a big piece of it alongside some more little ones is visible when Joker asks for his phone call. (01:30:00 - 01:33:05)
Continuity mistake: On lower 5th, the SWAT driver yells lock and load. When he does he has 2 cable ties in loops on his left hand side of his jacket that are exactly equal in size, both the loop and the "tail." We see outside the van for a few seconds. When the camera returns to the driver, the loops are now different sizes as evident by one having a longer "tail". (01:14:00)
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 ★