In our final deal on how to tackle 6-1 fits, South takes a shot at four hearts based on the strength of his heart intermediates. The defense kicks off with two rounds of spades, forcing the South hand.
Declarer has to avoid three trump losers. The percentage play is to start with a heart to the jack and then lead out the king. In this way, he picks up Q-x onside as well as 10-x in either hand. By contrast, playing up to the heart king first would not be sufficient if East had begun with A-x. Even though the king would score, you would still have three trump losers left.
So at trick three, South crosses to the club ace to play to the heart jack, which holds. He should now lead out the heart jack to pin an original holding of 10-x in either hand. East wins with the heart ace and continues the spade force, reducing declarer to equal trump length. If declarer were to drive out the heart queen now, East would play another spade, extracting South’s final trump and securing a two-trick set. Perhaps declarer should expect this after the fall of the heart 10 and East’s forcing defense. If he correctly divines that East has both remaining hearts, he should consider running the club suit through East instead.
The clubs would then function as trump substitutes. If East ruffs in at any point, declarer can overruff, cross to the diamond king and continue clubs. If East discards all the way, it does him no good — now all of declarer’s diamonds go away.