What is Adm. Grace Hopper's "Rope Trick"?

MinusOne submitted this odd question "The recenct discussion of female geeks brought up the name of Admiral Grace Hopper. She holds the distinction of being the geek with the most expensive thing named in her honor: a multi-billion dollar U.S. warship! I have heard that she had a trick she used to do with a piece of wire that somehow demonstrated what the speed of light was, or some similar thing. As a geek with a decidedly historical bent, I have always wondered what this 'rope trick' was, how it worked, and what exactly it demonstrated." Wouldn't mind knowing that one, myself...but how could it have been a 'Rope Trick' if she was using wires? :)