How does duck tape work?

It isn't duck tape, it is Duct Tape. Tape that is used to join and seal duct work. Very sticky tape.

Duct tape also has a layer of interconnecting fibers to provide strength and rigidity. These fibers are also designed to be very strong, but if duct tape is ripped from the side at a 90-degree angle, the fibers break very easily resulting in easy usability without scissors or a knife.