Dealer South. Both Vul

                                    ♠          A J 7 2

                                    ♥          A K 9 4

                                    ♦          9 6 3

                                    ♣          8 6

            ♠          8 6 5 3                                     ♠          —

            ♥          Q 10 7 6 5 2                ♥          J

            ♦          7 5                               ♦          J 8 4 2

            ♣          2                                  ♣          K Q J 10 7 5 4 3

                                    ♠          K Q 10 9 4

                                    ♥          8 3

                                    ♦          A K Q 10

                                    ♣          A 9

3NT agrees Spade

(*)    Cue

West led the two of clubs, in view of East’s pre-emptive strike it seems to be a fairly  obvious singleton Declarer could only count eleven tricks but saw that the main possibility to address the shortfall was four  possible tricks from diamonds and a club ruff in dummy. However this  chance all but disappeared when East threw a club  under the king of trumps

Declarer assumed that East began with eight clubs Consequently, as East could have at most five cards in one of the red suits, it was safe to cash the red aces When East followed to both of these cards he could have at most four cards in any red suit, making it safe to cash the red kings. When declarer cashed the king of diamonds and East produced a second small one rather than the hoped for jack, he could no longer rely on ruffing a club in dummy for the twelfth trick Instead he would have to fall back on ruffing two hearts in hand to score an extra trick. (now a dummy reversal)

After cashing the king of hearts and noting East's club  discard, marking an original 0:1:4:8 distribution on his right, declarer ruffed a heart with the nine of trumps Dummy was entered by playing the queen of trump to dummy's ace and another heart was ruffed with  the ten of trumps.

By now the importance of unblocking in trumps should be clear; the four of trumps was led and the seven finessed, providing the vital entry to draw West's remaining trumps. After throwing his club  loser on the jack of trumps everyone had two cards left. Dummy had a club and a diamond and declarer the queen and ten of diamonds. East was well and truly squeezed and could not keep  two  Diamonds and a club so released a club. Declarer was counting very carefully and was now able to cash dummy’s lowly winning Club

You can also play out the following hand  to see how the hand pans out. (You need bridgvu.exe programme  and it is available free from   http://www.haroldschogger.videoes.html or BridgeBase programme on your machine already to view it )

alternatively watch the film of the hand being played out by going to www.haroldschogger.com/movies2.htm