Untitled XV

Fundamentally, this is about the timing for the unblock in clubs. West's only entry to run his clubs is in spades. East/West must unblock the club ace before the spade king is taken.

If North/South are declaring, then, the defense immediately takes the club ace, and then East leads a low spade. If South ducks, the defense has five clubs, a spade and the ♥ A. But if South takes the trick, he is no better off.

If East/West are declaring, then South wins a diamond and immediately leads a low spade to the queen. If West wins, his hand is finished, and declarer leads a spade to dummy's queen, leads a heart which East must duck, and then takes all his winners, exiting with the fourth spade. East gets a long diamond, a spade, and the ♣ A, but must eventually lead from the ♥ A, giving South a second heart trick. So West must duck the spade, North wins the queen, and leads a heart. If East flies the ace, that sets up seven tricks for South (three diamonds, two spades, and two hearts.)

So East must duck. South takes the rest of his diamonds, then plays ace and a spade, again killing West's entry before the clubs are unblocked.