Nearly Perfect

[OliverC]

This was an instructive hand on which I was Dummy, playing with a pick-up Partner.

NS Game, Dealer East

Bidding
East     South     West     North
No       1         No        2
2      3         3       4
4      No          No        5
All Pass

North (Dummy)
95
AQ94
J764
AKQ

3 led

South
J3
K10872
AK5
1062

Declarer won the opening lead, drew trumps in 3 rounds (East showed out on the second round and then discarded another Club and a small Diamond), cashed 2 more rounds of Clubs (East showed out on the second round and discarded 2 small Spades), and then exited with a Spade. East won the Ace, and then led the Queen, which held the trick and switched to the 10. Declarer allowed that to run round toDummy's Jack - 11 tricks - well done!

...but not really well done. By the point at which the trumps are drawn and the Clubs cashed, what do we know about the hand? East is known to have started with 10 cards between Diamonds and Spades (they've shown up with a doubleton Club and a singleton Heart). West supported Spades on what must be fairly flimsy values (and so probably has 4 of them), and critically, East did not open a Weak 2 in Spades. All of that should suggest to Declarer that East was 5152 shape rather than 6142 shape.

Starting to see the point here? Declarer must cash exactly one top Diamond before exiting in Spades, in order to remove West's singleton Diamond. Now the defence really are helpless and endplayed, and we know the  10 lead is going to run round to the Jack. On Declarer's actual line (especially when South's Jack of Spades appears on the second round of the suit, West can overtake (and ought to do so) in order to lead a Diamond through Dummy. Removing West's singleton Diamond takes that option away.

If it turns out that East simply doesn't like opening weak 2's and really was 6142 shape, you are no worse off, but you've given yourself that extra chance of making this.