When your partner leads to a trick and you are third person to play to that trick, a useful guideline is third hand High. When you are the last player on your side to contribute a card to a trick, you want to try and win the trick for your side, if possible; otherwise you want to help promote winners in your partner’s hand.
You need to play as high a card as necessary to try to win the trick. With a choice of equal (touching) cards, play the lowest. Holding queen, jack, 3 — play the jack —as high as necessary. When partner watches what Declarer puts on the jack to win the trick ( Declarer plays the ace) —-partner will know you must have the queen as well as the jack!
If partner leads a high card which will win the trick, you don’t need to play a higher card unless you have to unblock the suit by overtaking partner’s card. Consider this scenario, partner leads the King vs a no trump contract, this lead suggests partner has the queen and jack and maybe more cards in that suit. You are sitting with the ace, and a little card in P’s led suit. You must play the ace and return the small card to get back into partner’s hand. By unblocking the suit your side will be able to run the suit!
Another situation where you only play as high as necessary is the following: if DUMMY, 2nd hand has a high card that is not played (queen for instance) — and you have the King, 10, 4 — play the 10 which might win the trick! Keep the queen on the board trapped!!!!