Pay up or pay off?

有人问我，如何区分 pay up 和 pay off. Here are the explanations from dictionary.com: pay off, a. to pay (someone) everything that is due that person, esp. to do so and discharge from one's employ. b. to pay (a debt) in full. c. Informal . to bribe. d. to retaliate upon or punish. e. Nautical . to fall off to leeward. f. to result in success or failure: The risk paid off handsomely. pay up, a. to pay fully. b. to pay on demand: The gangsters used threats of violence to force the shopkeepers to pay up. 其实这两个 phrases 除了看上去相似，含义非常不同