What is a Morton's Fork?

Cardinal John Morton was the Archbishop of Canterbury during the reign of King Henry VII. His tax policy, later known as Morton's Fork, replenished the royal treasury. Morton's reasoning was this:

If the subject is seen to live frugally, tell him because he is clearly a money saver of great ability he can afford to give generously to the King. If, however, the subject lives a life of great extravagance, tell him he, too, can afford to give largely, the proof of his opulence being evident in his expenditure.

               ♠ A 9
               ♥ K 7 6 4
               ♦ A 5
               ♣ A J 10 5 3
♠ J 10 3                     ♠ Q 9 6 5 2
♥ A J 10 9 8 3 2             ♥ ---
♦ 10 6                       ♦ J 9 8 2
♣ 7                          ♣ 9 8 4 2
               ♠ K 7 4
               ♥ Q 5
               ♦ K Q 7 4 3
               ♣ K Q 6

South opened 1NT, West overcalled 3♥ and North bid 6NT. West led the ♠J.

How was South to make 6NT. If diamonds split, he would have plenty of tricks: five diamonds, five clubs and two top spades. Normally a 3-3 split is around 35.5%, but after West's preempt, it is even less than that.

South found the answer. He won the opening lead in hand, and advanced the ♥5. West was caught in a Morton's Fork Coup.

If West took his ace, that would give declarer two heart tricks, along with two spades, three diamonds and five clubs. Therefore, he ducked. Now South played four rounds of diamonds, conceding one to East. He won East's exit and had two spades, one heart, four diamonds and five clubs for 12 tricks.