Blocking and Unblocking by Terence Reese & Roger Trézel
Never play a card automatically. Always look ahead and consider what is likely to happen. This salutary advice will help you to make the right play on a hand such as the following:
 J 9 8 2 J 9 4 A 9 8 6 4 3 |
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| Q 10 6 K 7 6 3 2 J 7 3 7 5 |
K 7 5 A 8 5 Q 6 4 2 K 9 8 |
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| A 4 3 Q 10 K 10 5 A Q J 10 2 |
You are South, playing in 3NT. West leads the 3 and East plays the A. Beware lest, with less than your full attention on the game, you drop 10.
It is easy to foresee the consequence. Este will return 8, and when your Q appears, West maturally hold off, hoping that his partner will have an entry card and a third heart to lead.
In with the Q, you lead a diamond to the A and take a club finesse, which holds; but you have no second entry to the table, and when the A fails to drop the K there will be no play for the contract.
All you need to do on this hand is unblock the hearts by dropping the Q under the A. If West, as before, holds off the next heart you can overtake the 10 with the J and finesse twice in clubs.
Alternatively, West may take the K and look for tricks in another suit, but you can withstand attack in either spades or diamonds and meanwhile the hearts have not been established.