[Nipper1954]

If opener plays a low card toward five cards with the King, (not Trump suit) if you have 3+ cards in suit led, with the Ace of that suit, should the Ace be played?

[smerriman]

There's no one-answer-fits-all; it may well be correct to jump up with the ace if declarer is trying to steal a trick, or there's an urgent need to switch / set up another suit before it's too late.

But generally, you would tend to play second hand low, as you risk giving away tricks for nothing otherwise (eg if declarer has the queen, or partner has Qx).