Too often in Bridge, players forget the odds percentage at breaking of suits and in a hurry sometimes often make a fallacious move when all the time a safe risk free play was available. This NS hand is the last in the series of 4 hands test of dummy play. If you got this right you are surely ranking with the shrewdly calculating players who not only understand the odds at Bridge but know the difference of advantage that lies in the alternate lines of play. Although this is a simple test of dummy play where nothing complex needs to be employed in the making of the contract, it never the less demonstrates the obvious blind spot from which even finest of the Bridge players suffer at times to fall from grace:
NS held the following cards:
Not quite strong enough for 2 NT opening, which required precisely a minimum of 20H.C.P, compared to the 19 held. North began with an opening of 1C. As south many would bid 1D so that the stronger hand, should be the one to be bidding NT and playing as declarer with the advantage of the lead in dummy's weaker suits going into whatever strength of tenaces declarer holds. But here, south thinking he could express his whole balanced hand of 7HCPs in one bid, thought it wise to respond INT with the result that north had to become dummy in a contract of 3NT which north bid in no time. The opening lead from west is the 5 of spades.
Now here your testing time in dummy play begins now. What are your plans for 3NT?
The strong dummy laid down is pretty good but the spade opening lead has raised the danger bar a bit higher. Counting your sure tricks you can see 3 in clubs (otherwise 4), 3 in diamonds if the queen is not trapped (otherwise 4), and 1 in hearts with QH not on (otherwise 2), along with one certain spade trick when the JS holds with east following low with 4 spades. 8 top tricks in plain sight is always a pleasing one, with the endless alternates of club breaking, diamond Q trapping and heart finesse on providing you with that one additional trick that you so desperately need.
Well, what is your next move on winning the JS? Ah yes, you are clever enough to go for the diamond finesse in the safe hand of west who cannot harm you in spades without giving you a spade trick and the contract. There you are, if you have started with the KD followed by a finesse of the JD, you are done for. Winning the diamond queen, west would lead the heart deuce cunningly to force you into the heart finesse option immediately before you know whether clubs are breaking or not. Here there is a do or die situation with south finding himself off balance. Naturally as south, if you finesse. You go down with a spade return sinking your makeable contract into the downing up pit going 2 down.
It is emphasized time and again that when such situations occur it is always useful to know the chances of an even break in a suit or a simple finesse, which is a 50-50 proposition. South could reject the heart finesse and go up with the AH with an aim to take 9 tricks by way of 3 diamonds, 1 spade, 1 heart and all 4 clubs relying on a 3-3 break. But you too as south must have read somewhere that a 3-3 break of 6 outstanding cards in a suit are against the odds. South, like you, figured a 36% initial break too low for consideration direction.
But he missed, like you, a vital inferential clue from the bidding or rather the absence of bidding by the defenders. True, the chances of a 3-3 club break was not much of a hope but the point to consider was that if west held, say Kx in hearts along with 4 clubs, would he not be in trouble when you play 2 more diamonds. Keeping a master club with the Kx of hearts, after 8 tricks played, say AKQ of clubs, 1 spades and 4 diamonds, west would per force have to part with a spade and be endplayed in hearts. Now do you see what should have been your decision at trick 2. Testing the club first before taking the diamonds finesse. As it is clubs were breaking 3-3. The declarer's mistake was not of calculation but a mistake in tactics.

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 North         South
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 J 10 3        K 9 6
    A Q       10 7 8
K 6 4 2     A J 10 5
A K Q 2        9 7 3
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