Source: Reading Eagle – 23 Mar 1977 The secret of good dummy play is good card reading. This consists mainly of diagnosing how the unseen cards are likely to be divided and then taking the proper measures to achieve the best possible result. Good defensive play likewise rests on good card reading. Dealer East N/S Vul
J 6 4 7 3 K Q 8 A J 9 6 4
10 9 5 Q 10 8 6 2 7 5 3 7 2 K 8 2 A J 4 9 6 4 2 K 5 3
A Q 7 3 K 9 5 A J 10 Q 10 8
West North East South
Pass 1NT
Pass 3NT Pass Pass
For a fine example, consider that deal where West leads the six of hearts and East plays the jack! It may seem odd for East to play the jack instead of the ace, but before we attempt to analyse the reason for his play let’s first look at its effect. As a practical matter, South has no choice but to win the jack with the king. From his viewpoint West  might have led a heart from a holding such as A-10-8-6-2 — in which case ducking the jack would produce rapid defeat. South then leads the queen of clubs, which loses to the king. East thereupon plays the ace and another heart to put the contract down one. Note that if East plays the ace of hearts at trick one declarer makes the contract. South ducks the heart continuation, wins the third heart with the king, loses a club finesse, but eventually scores at least nine tricks. East, jack of hearts play is unquestionably correct. East sees 11 points in dummy and 11 points in his own hand and, by assuming at South has at least 16 points for his opening trump bid, he knows that his partner has at most 2 points, East’s only real hope, therefore, is that West led from a holding consisting of five or six hearts to the queen. Accordingly, he plays the jack at trick one. It is true that in the actual deal South can make the contract by ducking the jack, but it is much easier to make this play in cold print than in the heat of battle.