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.