Playing on autopilot

Here's another excellent deal from the International Bridge Press Association newsletter:

                ♠ 7 6
                ♥ 10 6 5
                ♦ A J 10 8 7
                ♣ A J 7
==
                ♠ A K 8 4
                ♥ A K Q
                ♦ K 5 2
                ♣ K Q 5

South was the declarer in 6NT. West led the ♠Q. South won with the ace and played the ♦K -- the contract could no longer be made. Here are all four hands:

                ♠ 7 6
                ♥ 10 6 5
                ♦ A J 10 8 7
                ♣ A J 7
♠ Q J 10 2                     ♠ 9 5 3
♥ J 9 8 4                      ♥ 7 3 2
♦ ---                          ♦ Q 9 6 4 3
♣ 10 9 6 4 3                   ♣ 8 2
                ♠ A K 8 4
                ♥ A K Q
                ♦ K 5 2
                ♣ K Q 5

Playing the ♦K seems automatic, doesn't it?

Declarer has eight tricks outside the diamond suit. To ensure the contract, he needs to score four diamond tricks. To do that, South should play a low diamond at trick two. If West follows, declarer can insert the ♦J. If, as is the case here, West shows out, declarer again plays the ♦J.

If East takes the queen, declarer wins the return, crosses to the ♣J and advances the ♦7. Whether East covers or not, South has four diamond tricks and claims his slam.