The awesome power of the dog’s sense of smell is almost beyond belief. Because we can never completely understand how canine scenting processes work, we don’t ‘teach’ dogs how to track – we provide some guidance, and a framework in which they are able to learn what is expected, and they go on to show us how incredible their noses really are. At a tracking test, the dog follows an unmarked track laid down some time before by a stranger. The dog must follow it to find one or more articles dropped along the way and at the end. He wears a harness, and is attached to his handler by a long line, but the handler is    pretty much just along for the ride. The dog is the one that knows where he’s going.
(Description courtesy of Canadian Kennel Club)