Defence: When to cover an honour with an honour?

When declarer/dummy leads an honour, the default position is to cover the honour with your higher honour, to draw two of declarer’s high cards for one of yours:

If you as East are too mean to play ♠K on dummy’s ♠Q, ♠Q will win and ♠2 to ♠J will follow, enabling declarer to score all three tricks. Instead, cover ♠Q with ♠K and declarer can win ♠A and ♠J, but not ♠Q.

Do not always cover an honour with an honour as a reflect reaction, however. If there is no chance of promoting lower cards for your side (effectively the point of covering an honour with an honour), then don’t bother.
 

If ♠Q is led, you as East should play ♠4. You can see that covering ♠Q with ♠K will promote ♠J109. By playing ♠4 on ♠Q, declarer’s ♠A will win the second round perforce (because he only has two spades) and your ♠K will be a third-round winner.

So cover an honour with an honour if you think you can promote lower cards for your side. Otherwise don’t.