Against South's 4 contract, West found his best lead of the spade ace and continued with a spade to the king when his partner signaled encouragement. Declarer ruffed, played the ace of trumps, and paused when East discarded a diamond.
Now a diamond was led to the queen and ace, and East eagerly continued the tap by returning the spade jack, declarer ruffing again. Two rounds of clubs were cashed, followed by the diamond king. This was now the situation:
When declarer now led the 10, West was helpless. If he discarded his spade, declarer could pitch a black card from dummy and then play his last diamond, forcing West to either ruff and get overruffed, or to ruff with the queen and concede the rest. If West ruffed high, declarer could either ruff the spade return in dummy or win the heart return in hand to draw trumps. In practice, West ruffed with the 4; declarer overruffed in dummy, ruffed a spade, and played the 9. West, forced to ruff, had to lead from the Q6 into declarer's K10 at trick 12. Making 4.
Had declarer not cashed two rounds of clubs before playing off his diamond winners, West could have pitched his clubs and eventually scored a club ruff to go with his heart queen for one down.
It is also interesting to note that had East ducked his diamond ace, declarer's timing is thrown off and careful defense always sets the contract.