Source: www.qldbridge.com 
This deal comes from the Vanderbilt, one of North America’s three main team events. It was played by Eric Greco.
[1] 3 card spade support, good §s and game force
[2] Key card ask in clubs
[3] Three Key Cards
[4] Asking for Q
[5] I have the queen and no outside kings
West leads a low diamond against your grand slam. Obviously, you win the ace of diamonds. What do you play next? Greco made a key play at trick two – he trumped a diamond. What’s the difference, did you ask? Take a look. The A-K revealed the bad break. Now a heart to the ace, diamond ruff, heart to the king, diamond ruff, ace of spades, spade to the king and the queen of hearts, pitching the third spade. There are wo cards left, declarer has Q-10 and dummy is on lead. East is caught in a trump coup. None of this would have been possible if a diamond wasn’t trumped at trick two.
Again on the next deal, many points also hung on declarer’s play at trick two. This deal, from the Bermuda Bowl in Paris, was reported by Ted Chadwick in Australian Bridge Magazine. East opened one diamond and Peter Fordham quickly found himself in four spades. West led the Q. East took the ace and switched to the 9. Fordham, perforce, played his 10 and West played low. What would you do?
Fordham played a low spade to the queen and East’s king. East cashed the A so now Fordham had to guess who had the jack of spades. He decided to play East for K-J-x and went down when they turned out to be 2-2.
At the other table, the declarer overtook the 10 to cash two more clubs, shedding his losing diamonds, and played a heart. East ducked his ace and so declarer crossruffed madly and made 12 tricks when one of the defenders played a high trump. Fordham’s play might be a winner sometimes but not on this lie of the cards.