A live metaphor is one that evokes in the reader a mental picture of the imagery of its origin; a dead one does not. If we write "the situation is in hand" and " he has taken the bit between his teeth", we are in both going to horsemanship for our metaphor. But to most readers the first would be a dead metaphor, and the sentence would have no different impact from "the situation is under control"; the second would be a live one, calling up, however faintly and momentarily, the picture of a runaway horse.