I am having trouble understanding the Khan academy video of a hair dryer teardown here: https://www.youtube.com/watch?v=gFFvaLzhYew. In this video, the lecturer explains how the safety plug works (so you don't fry yourself if you drop the hair dryer in the sink). At 3:50, the lecturer says that the copper coil can sense when the neutral and hot wires are charged differently, and it then alerts the control chip to turn off the power. The explanation of the operation of the coil was very vague. The picture below points to the part I'm talking about:
What is the copper coil that he shows in the video? How does it work? Is there a more specialized name for the coil when used like this ("safety coil" or "difference coil", etc.)? I ask for the name because I couldn't find an explanation of this via Google, probably because I didn't know what to search for.