After the dummy has gone down the bidding has pinpointed every single point that you cannot see. Unless South is a complete cretin South must have good Hearts and 13-15 points for his 3NT bid. Dummy has17 and you have 8 which means your partner, the other defender, has no more than 2 points. With this knowledge if declarer happens to have a 4 card club suit you can now count declarer’s hand for 2/3 Hearts to come, 6 Diamonds and 4 possible Clubs. To stop an easy 12 tricks you must cash your two Aces at tricks 1 and 2 to obtain a well above   average  Match point score

(NB if you don’t take your Heart Ace straight away and declarer plays a second Heart remember to cash your Spade Ace)

You can also view  watch the following movie to see how the hand pans out. (You need bridgvu.exe programme  or BridgeBase programme on your machine already to view it ) and it is available free from

http://www.haroldschogger.com/videos.htm

 alternatively watch the film www.haroldschogger.com/movies2.htm