Can South make six diamonds today after the club jack lead?
One chance is to find one defender with a miraculous A-K doubleton in hearts and ruff them out, but a squeeze offers a better chance.
It is best to duck the opening lead in dummy, hopefully leaving East with the sole club guard. Declarer ruffs and must then play a heart. Say West covers with the 10 and declarer puts the queen on, hoping to leave West with the only guard in that suit.
East takes the trick with the ace and does what? A top club return seems safe enough but is actually far from it! Declarer ruffs high, draws trumps ending in dummy and ruffs a heart high. He then runs trumps, and as he leads the final one, he has three spades in hand facing K-x in spades, with the heart eight and club queen in dummy. West must discard a spade to keep the heart king. The heart eight goes from dummy, and now East can either let go of the club ace or unguard the spade suit. If he does the latter, declarer’s spade three scores the 12th trick.
A red-suit play at trick three is also ineffective. To defeat the slam, East must shift to a spade at trick three, which is safe enough from his Q-10-9 sequence. By attacking the double menace, he destroys the communications necessary for declarer to bring off his double squeeze.
Without that same spade shift, it would not help West to take the first round of hearts with the king. Declarer could cross to the diamond ace and later isolate the heart menace by continuing with the heart queen.