Here is the full hand. West opened 1
♥ passed to South who jumped to 3
♠ and was raised to game, doubled by East.
When East followed with the 3 at trick 1, it suggested that South had a singleton. East must clearly have doubled on good spades (remember that he passed 1
♥, so he has less than 6 hcp), presumably two certain tricks, but for his jump to the 3-level, South is likely to have 7 spades.
What you must be careful about is that you don't let declarer score too many ruffs with small trumps, thus reducing his trump length. Let us see what happens if you carelessly ("automatically"?) continue with a high heart at trick 2.
South ruffs and gets the bad news when he cashes
♠A. He enters dummy with a club and ruffs another heart. Back to dummy with a club to the king, and he pitches a diamond on
♣Q. He now leads dummy's last heart, and East can't gain anything by ruffing. If he does, declarer just pitches another diamond loser ("loser on loser") and is home directly.
So East pitches a diamond, South ruffs, and dummy is entered again with a diamond to the ace. This is the 4-card ending after South has taken 8 of the first 9 tricks:
When dummy's
♣2 is led, East is exposed to a "coup en passant", and no matter what he does, declarer will score one of his small trumps and the contract.
Now, could the contract have been defeated? Yes, a club switch at trick 2 knocks out one of dummy's entries prematurely. South loses the tempo and can't achieve the proper ending. The result will be 1 down. Feel free to try yourself if you don't believe me.
Roland
S: 4S doubled
Lead: Ace of hearts