In high school we were all taught that electricity only flows in a loop. But it seems that one can send a "pulse" down an open conductor. This is used in a [time-domain reflectometer][1]. How is this possible?

How can you send electricity down an open conductor? What didn't they explain in high school that allows this to be possible?

  [1]: https://en.wikipedia.org/wiki/Time-domain_reflectometer