West led the club king against four hearts and switched to a spade, won by dummy’s ace. Declarer naturally started on trumps, leading the queen. East ducked the first round but then won the second before returning a spade. He had accurately diagnosed that declarer was out of clubs; otherwise, he would have been trying to score ruffs in dummy.
That spelled the end of declarer’s chances. He had to win the spade return and either play a third spade, whereupon East would ruff and shift to a diamond to knock out dummy’s ace, or open the frozen diamonds, allowing West to win with the king and return the suit to set up a second winner for the defense.
Declarer made a mistake when he led that second round of trumps, albeit a subtle one. With sure diamond, club and heart losers, he should have concluded that he needed spades to be 2-2 to stand any chance. If that was the case, there was no danger in cashing a second round of spades before continuing trumps. This maneuver is known as a Dentist’s Coup — preventing the defenders from endplaying the dummy. Yes, it might lead to extra undertricks, but it would still be an excellent investment.
If declarer had extracted the spades, East would have won the second trump trick and shifted to a diamond, but now it would be the defenders opening the frozen suit, not declarer. South would simply follow low from hand to ensure only one diamond loser. Then he could take West’s king with the ace and play another diamond, hoping to avoid a diamond ruff.