Consider this situation:
 A 2 K J 6 Q 9 8 A 10 8 7 4 |
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| 10 9 6 3 A Q K J 10 6 Q J 2 |
The contract is: 3NT
Lead: 7
What would you do? It is probable that East holds one honour in the spade suit, for had West held all three he would have held a sequence from which he would probably have led his highest honour.
If West has only a four suit of spades, South cannot prevent West from making them by a hold up in any case, but if West has five spades, the assumption is that East has two spades, one of which is an honour.
The ace of spade should therefore be played on the first round. East has to choose between blocking the suit with his honour, or of promoting South’s 10 to an eventual winner should he throw his honour under the ace.
Supose the adverse hand were:
| A 2 K J 6 Q 9 8 A 10 8 7 4 |
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| K J 8 7 5 9 3 2 A 7 4 5 3 |
Q 4 10 8 7 5 4 5 3 2 K 9 6 |
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| 10 9 6 3 A Q K J 10 6 Q J 2 |
On the lead of 7, South will make his contract if he goes right up with his ace; not so if he plays his 2. In the latter case East would return the lead and clear the suit before West’s ace of diamonds had been extracted.
When the first trick is won with dummy’s ace, East cannot profitably throw his queen under it. South must not, of course, now enter his own hand by the ace of hearts and finesse the clubs, he must at once attack the ace of diamonds and continue leading diamonds until the ace is played.
For it not, when East gains the lead with his K, he will, after making his queen of spades, enable his partner to make the rest of the suit by putting him the lead by playing diamond.