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.