It made a strange, homespun kind of sense. Heat expands, cold contracts. The clog was likely a greasy, fibrous plug of paper and other, less mentionable things. Heat might soften it, loosen its grip, let gravity do the rest.
The water should be "hot tap water" temperature or slightly hotter—roughly 120°F to 140°F (think of the temperature of a very hot bath or coffee that has been sitting out for 20 minutes). It should be hot to the touch but not scalding.