The metal coathanger technique is a form of dousing, a very ancient "art" that has been used to find water for a long time. Strangely enough, the technique is so common that the United States Geological Survey actually did a scientific study of the method, in an effort to determine if it was useful for locating water wells, finding buried pipes etc. There conclusion was that there was no evidence it was effective. But of course, USGS is a government agency, so there are many people who don't believe anything the government has to say.
Other studies have concluded that successful "dousers" are actually using a method similar to the Ouija Board, namely they subconsciously manipulate the rods based on their intuition about where the water lines might be. If they are clever (as in the Mentalist on TV), they may locate the line, and after several successful attempts, may come to believe in the power of the rods. Personally, I am very skeptical, as I am about psychics, mentalists, astrologers, precision dice throwers, and a variety of similar "professions".