You are sitting East. North, in 3rd seat, opens 1NT. You overcall 2D, passed around to North, who bids 2H, ending the auction. You lead the Diamond Ace, partner playing the Jack, then continue with the Diamond King, fetching the 2 from partner and the Queen from declarer. What do you do now?
By cashing the A and K you have established Dummy’s Diamond 10 as a winner for a late discard. You should play a 3rd diamond. You know North can overruff your partner but you will prevent him from using that discard later. You’ll win 4 tricks: 2 diamonds, a club, and a spade, holding the contract to 3. If you play on anything else you allow declarer to make 4.