The Law of Total Tricks, which players use to make competitive decisions, states that the total number of trumps both sides have in their best suit equals the number of tricks available. Today's North-South have 10 diamonds, East-West have nine spades; 19 tricks should be won at spade and diamond contracts.
One LOTT problem is you can't always tell how many trumps your opponents have. When South's bid of three diamonds came around to East, he might have tried three spades if he'd known North-South had a 10-card fit. (If three spades failed, North-South could presumably have made five diamonds.) But East passed. He had a junky hand and couldn't know that North had four-card diamond support.
East-West could have made three spades, but they went plus on defense. South couldn't - or didn't - make even three diamonds. West shifted to the ten of clubs at Trick Two: queen, king, ace. South took the A-K of trumps, ruffed a spade, led a club to the jack, and ruffed a spade. He exited with a club, but East won with the eight and led the ten of hearts. Down one.
South can set up an end play if he ducks West's ten of clubs. If West leads a trump next, South takes the A-K, then lets the queen of clubs ride. He ruffs a spade, takes the ace of clubs, leads a trump to dummy, and returns the jack of spades, pitching a heart. West wins but must concede the contract one way or another.
West could prevail by leading a trump or a low club at the second trick.