We overload standard 14/2 wire commonly used in Canadian and American houses to find out how much current it can handle before it melts and burns up. Spoiler: It handles way more than its 15 Amp current rating as one would hope for a good safety margin. Of course it should never normally to subjected to anything beyond than its standard current rating.
See the FAQ video about this and the other over-current tests here: • Overcurrent Tests FAQ