When your opponents voluntarily undertake a slam, you should not double them unless you are absolutely certain you can defeat the contract. This is especially true if your double gives declarer information that might allow him to make a contract he would not otherwise make.
In the present case, for example, West clearly should not have doubled six notrump. In doing so, he revealed to declarer where the missing high cards were located.
Armed with this knowledge, South made the slam by adopting a somewhat unnatural line of play. He won the spade lead with the queen, led a low diamond to dummy’s jack and returned a diamond to the queen. After West showed out on the second round, leaving South with just 11 tricks, declarer cashed the A-K of diamonds and dummy’s A-K-J-10 of spades to produce this position: When South next cashed dummy’s last spade and discarded the jack of hearts, West found himself in dire straits. He could not afford to discard the jack of clubs, which would have given declarer two club tricks instead of one, so he discarded the queen of hearts instead. This did not help West at all, because South simply led a heart to the king and won the last two tricks with the queen and ace of clubs.