South ended in Four Hearts after West had opened the bidding with One Spade, which was followed by two passes.
West led the king of clubs which declarer ducked, and won the club continuation. His first idea was that if East held J 10 bare or J 10 x in spades, he would be home.
However, when he played a trump and West showed out he knew that West had at least five spades, so looking to find East with J 10 x had to be crossed off the list of possibilities. (East could just have the ace of diamonds for his initial pass, but South felt that this had to be the last resort.)
But now declarer appreciated that other possible holdings by East in the spade suit, such as a singleton or doubleton jack or ten of spades would lead to success. South drew the last trump, then led the eight of spades from hand. (Not the queen, for West could take this and get off lead with another spade.)
West had to play low and dummy's king won the trick. Declarer followed with a small spade towards the queen and was delighted to see the jack from East. West, in with the ace, had either to open the diamonds, concede a ruff and discard, or play a spade, which would establish a trick for dummy's nine for a diamond discard.
4K 9 6 3
!A 10 9 5
#9 7 4
4A 10 5 4 2 4J 7
!Void !8 3
#A Q 3 2 #J 10 6 5
2K Q 10 5 2J 9 7 3 2
!K Q J 7 6 4 2
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