Superficially, it appears that you need successful finesses in spades and diamonds. However, there is an extra chance when the full deal is:
After drawing trumps, you should cash the ♣K, ♣A and ♣Q in that order. When the clubs are 3-3, you can make certain of the contract by leading a spade, covering East’s card. This endplays West, forcing him to lead into one of your tenaces.
If you draw trumps and cash the clubs ending in hand, you can no longer make the contract on the above layout – there are not enough entries to extract a 12th trick. If East had one of the kings you could survive only by guessing which king he held, a dubious prospect at best.
When East has four or more clubs you can adopt the same approach in spades, covering the card that East plays. Again, this endplays West and allows you to make the contract when West began with a hand such as
♠K 5 4 2 ♥9 8 5 ♦J 8 7 5 ♣10 6.
While you will need the diamond finesse, you can take advantage of East holding either the ♠K or the ♠J and ♠10.
When West has four clubs, you need both the ♠K and ♦K onside. This means you should finesse the ♠Q on the first round of the suit. A point to appreciate is that if you play the ♠ 9 on the first round, you will fail when West began with
♠J 5 2 ♥9 8 5 ♦J 8 7 ♣10 8 6 5
He will take the ♠9 with the jack and play his ♣10. After ruffing this in dummy you will no longer have the entries to finesses both queens.
