A.N. Onymous once said, "An expert is one who knows more and more about less and less."
But that is why she or he is an expert -- specialization. When you are the declarer at the bridge table, though, the more you know about one defender's hand, the more you can deduce about the other's.
In today's deal, South is pushed into five diamonds. West starts the defense with his two top spades. After ruffing the second on the board, how should South continue?
West's jump to four spades promised a good eight-card suit and some 6-10 high-card points. Now North had an awkward decision. Doubling and taking the money (assuming South passed, which he presumably would have done here) was feasible. That would have netted only 200 from down one. Instead North, liking his offensive potential, competed with five diamonds.
Declarer has to draw trumps without loss. If the opponents had not bid, cashing the ace and the king would be mathematically slightly superior (by about two percent) to cashing the ace and finessing on the second round (nine never). But now the odds have changed. West has only five spaces for the diamond queen, while East has 11. Finessing on round two is now almost twice as good a play.
Even better is to start with dummy's jack. If East plays low smoothly, South puts up his ace. If the queen drops, fine; if not, declarer returns to the board with a club and plays a diamond to his 10. The plus of starting with the jack comes when East erroneously covers while holding all four trumps.