Defensive problems are intricately women into the overall bidding strata and then arise at a critical point of time where the do or die decision is suddenly thrust upon the defender. Not many defenders think too far or deep. But those who do make it a point to put a little sense of logic behind their moves. Without it the efforts at deep concentration would be in vain as happened in this illustration taken from a National Swiss Team Championship held at Liverpool, dealer being south with no one vulnerable, the bidding proceeded as under:-
Although the bidding was a bid archaic for no negative double was put in, with the players playing no 5 card major opening. South's 2NT in this sequence of bidding contained minimum 18 HCPS.
In this 3NT contract, let us now place you in west's seat with the following hand:

K 8
K Q 9 8 6 4
10 8 3 2
3

West's opening lead of QH being mentioned suggested a suit headed by KQ9 and as such requested partner to drop 10H, if he had it and if partner held say J10 of hearts, he would part with JH. As the cards lay, east dropped the 10H when the following dummy surfaced:

Q 6
5 3
K J 7 6
K 10 7 5 4

When south played the deuce of hearts it was quite clear to west that now south held AJ of hearts, making it impossible for him to continue the suit without giving away a trick. The critical point of defence has been reached. As west, what is your next move?
Let us think what could be the best option by trying to eliminate other leads on trick 2. Although west could count on his partner holding at least 4 clubs to an honour the club lead still looks a bit risky as it could easily trap partner's honour under declarer's strong holding in clubs. What have you decided?
As you look at the alternates left there is a spade shift that on the face of it with a holding of K8 seemed almost out of question. That left only diamonds and with west's holding of 10 8 3 2, in the actual play west decided to shift to diamonds by playing the deuce of diamonds.
Did you also resort to the diamond switch? If you did I am afraid your defence would collapse as it did in the actual deal.
For these were the full hands:
Looking at all 4 hands, can you now spot the winning defence shift? For you can see that on west's 2D hoping to find entry into easts hand for him to return a heart through the declarer, south won with the QD and lost a club finesse to east's queen. East switched to a heart but declarer was by now home with heart trick, 4 diamond tricks and 4 club trick for a nice tally of 9 tricks to make 3NT.
The winning defence is a subtle shift to the 8S from king doubleton. As the cards lay east wins with the AS to return the 7H. Now with AH south can only take 8 tricks even if he takes the winning club finesse with west holding a sure spade entry of the KS to run the winning hearts.
Was the spade switch on a holding of KS too impractical? On the contrary it was the only logical defence. For it east held say AH as west supposed he was unlikely to hold the AS. Say east on winning the AD (if so) leads back a heart, south ducks once and then can safely finesse clubs into east's hand and east has no more hearts. Then south with 1 spade trick, 1 heart, 4 clubs and 3 diamonds can easily net in 9 tricks for his contract of 3NT. So a spade switch of 8S at trick 2 by you as west would have been a defence par excellence.

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S     W     N          E
1S   2H    2S          P
2NT   P   3NT   All Pass
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North        West         East           South
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Q 6          K 8          A 9 4 3 2   J 10 7 5
5 3          K Q 9 8 6 4  10 7           A J 2
K J 7 6      10 8 3 2     9 4            A Q 5
K 10 7 5 4   3            Q 9 8 2        A J 6
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