After two passes East opened One Club and South pre-empted, rather wildly, with Four Hearts. West doubled and all passed. West cashed the ace and king of spades and switched to the queen of diamonds which declarer mistakenly failed to cover in dummy.
Still on lead, West led a trump and East covered dummy's three with his six - a good play. Now declarer could neither gain access to the ace of clubs nor ruff a diamond and the resultant penalty was 800 points. If the queen of diamon-ds had been covered, East's best defence would have been to lead a trump, but then the seven of hearts would become an entry to dummy and the ace of clubs enjoyed.
It is interesting to speculate on the fate of a possible Four Spade contract by West, North would like to do two things - cash the ace of clubs and give his partner a club ruff, and push a heart through dummy's king. He cannot do both. Should he start with the seven of hearts, it would be vital for declarer to cover with dummy's king - apparently a futile play, but it leaves South on lead and so avoids a club ruff.