WHEN an honour is led and you have a doubleton honour in the suit it is generally right to cover – unless the suit in question is trump. The purpose of covering is to promote tricks for cards of lower rank, and you cannot do that when both of you are short in the suit.
On this deal declarer's only chance was that an opponent would make a reflex unthinking cover. Both North and South liked their hands. North opened a strong 2H, and South made a positive response in spades, When he rebid the suit North was confident that he must have two of the top three honours. Blackwood does not help when you have a void, so he simply blasted slam. (A useful bid here would be 5S, asking partner to bid slam if he thought his trump were good enough.)
Declarer was disappointed in his dummy, but tried not to show it. He won the diamond lead, crossed to a top club, and advanced the jack of spades. East covered with the king, crashing his partner's ace, and there were recriminations. Poor East could not defend his play.
Declarer surely had at least six spades for his bidding, so partner could not have more than two. Covering the jack could not promote anything. If partner had Qx, even Q10 doubleton, there was always one trick, never more than one.
When declarer leads a trump honour from dummy he wants you to cover. Do not give him what he wants.