After the lead of an Honour card : 2nd player covers !
to promote a lower card to a winner (for his side)

  2nd player
Covers
 
    Lead
    A high card
3rd player
Wins if he can
  4th player
Usually plays low
(or wins if he can)
 

Principle
After the lead of an Honour card, 2nd player covers (plays a higher card) in order :

  1. to promote a lower card (J, 10 or 9) to a winner.

  2. or to reduce the number of tricks the Opponents can make in the suit.

Exception
When it is clear that there is nothing to promote on Defender's side.

  2nd player
♥ K 7 3
Declarer
Q 6 4
  Dummy
♥ A J 10 9

In such case 2nd player plays a low card in order to keep Declarer guessing.