CP3 - After the lead of a low card
2nd player plays low !
to maintain 4th player's advantage
  2nd player
Plays low
 
    Lead
    A low card
3rd player
Plays high
  4th player
Plays a higher card
(if he can)
 

Principle
The great advantage of the non-leading side is that one of their players will be the last (4th) player to play to the trick. His high card can therefore not be captured by the opponents.

Therefore whenever one side (either Declarer's or Defenders') leads a low card the 2nd player plays low to maintain this advantage !!

Whenever the 2nd player plays high he deprives his Partner (and his side) of this advantage.

Exceptions
Whenever 2nd player's Honour card would become unguarded (K x or Q x x) he must play his high card (otherwise it will be captured the next trick).

On rare occasions the 2nd player plays high in order to win the lead quickly, for example when he holds enough winners to defeat the contract, or (in a trump contract) to lead a suit his Partner can ruff.

 
CP4 - 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.