There’s an axiom on defense, “don’t finesse partner”. What does that mean? Here’s an example:
Board 8. Contract 1NT North
The contract is 1NT by North after an uncontested auction 1D-1S-1NT. East’s opening lead was the H6 to the 8 and K. The H2 was returned to the 9 and A. East shifted to her 4th best D (the 6) and West “finessed her partner”, i.e. she assumed that North had the Ace and East the Queen. She played the Jack and North took the Queen. The Club Q was onside and careless discarding led to a top board for North-South.
West’s error at trick 3 was not playing “Third Hand High”. Dummy did not have an honor in the suit. If North has the Ace and Queen it doesn’t matter which card you play. If North has only the Ace then East has the Queen. If North has the Queen (as was the case) then the King wins and the Jack return pins the 9 in dummy, establishing at least 3 tricks in the suit, 4 if East-West are patient.
The lesson: retain an honor when there’s a lower honor in Dummy. If there’s no honor in dummy then usually you should play Third Hand High with this holding, especially at NT.
I should point out an exception. Suppose the opening lead against a suit contract is a low diamond and you hold KJx. You should assume your partner doesn’t have the Ace. Play the Jack to see who has the Queen. This is only valid on opening lead against a suit contract. Mid-hand you should put the King up.