East contributed the king of spades under his partner's ace at trick one. West knew, from that play and the auction, that East could not have a high diamond, so he continued with a second spade. South ruffed in dummy and cashed the queen of hearts, revealing the bad split. South ran five trump winners, picking up East's jack in the process, leaving this position:
Declarer cashed his last heart and West was forced to discard the nine of diamonds to keep all of his clubs. Next came a club to the ace followed by the king of clubs as South discarded the jack of diamonds. On the queen of clubs, East had to keep a high spade so he discarded a diamond. South then discarded the 10 of spades. In the two-card ending, West was known to hold the high club and East the high spade, so the remaining diamonds had to be splitting 1-1. South led a diamond to the ace and scored up his slam!
Could the defense have prevailed? Yes! West could have shifted to a club at trick two. Try it for yourself.