One of the first and most basic rules of bridge is when you are declarer, "pull Trump as soon as possible." There are times, however, when you should not pull trump when playing the hand. Look at the following hand:
♠ K7 ♠ T98642
♥ 9874 ♥ AK
♦ 9 ♦ AJ5
♣ AKJT43 ♣ 98
West North East South
P 1S P
2C P 2S P
3H P 3N P
4S P P P
Opening lead: KD
Bidding Commentary: You don't like to open a hand in a major suit lacking the top four cards in the suit, but, on the other hand, East has a 6 card major and 12 HCP. It's hard not to open that hand, so East opened 1S.
West's 2C bid showed at least 5 Clubs and 10 points. East's only call is to rebid his 6 card suit. West's second response of 3H normally shows hearts stopped, denies support in East's suit and asks East to go to 3N with diamonds stopped, which East did. West then correctly went to 4S since she did have two spades and did not have hearts stopped. East should have bid 3S after west showed he had 6 spades and a minimum hand. Had she done so. West would pass with a minimum hand and realizing that East probably didn't have 3 card support.
Play Commentary: South's lead of the King of diamonds was unorthodox without also having the Queen. East took the Ace of diamonds. If he pulls trump immediately, he is pretty much assured of losing three trump plus two diamonds. He has to ruff his two losing diamonds before pulling trump, utilizing the two trump on the board to trump diamonds, even though one of them is the king. (I have to digress here a bit. As the cards lie, the queen of diamonds is onside, so if North gets in, she will probably take it, making East's Jack good, holding the diamond loss to one, for down one. However, declarer has no reason to believe that South's opening lead wasn't a normal lead of top connecting honors. Who would lead a bare king? So declarer will play the hand thinking that the queen is on his left and thinking he has two sure diamond losers unless he ruffs them.)
When you are setting up a cross ruff, you should take your winning tricks immediately. Since you are leaving trump in the hands of your opponents, you don't want them sluffing cards in the suits in which you have winning cards while you are cross ruffing, and ruffing in when you try to take them. As you ruff, one of the opponents will run out of the suit and can get rid of cards in the suit you want to win with your aces and kings. So after East ruffs the 7 of diamonds on the board, he gets back to his hand with the king of hearts, ruffs the last diamond with the king on the board and gets back to his hand with the Ace of hearts.
Still avoiding trump, he plays to the Ace and King of Clubs, thereby taking all his winning tricks outside of the trump suit. At this point he has taken the Ace of diamonds, AK of Hearts, AK of Clubs (seeing the Queen fall from his LHO) and ruffed two diamonds on the board, so he has taken 7 tricks. He knows that South is out of clubs, so he ruffs a Heart for his eighth trick, breathing easier to discover that hearts broke. Now he's got 8 tricks. He needs two more and here's the final layout:
♠ ♠ T9864
♥ 9 ♥
♣ JT43 ♣
Now he can pull trump (having no choice since that's all he has in his hand), and leads the Ten. South plays low and North takes her singleton Ace. North leads the Queen of diamonds. East must trump with the six because he can't afford for South to win anything but the Queen or Jack. South takes the Jack and leads the Queen of hearts, allowing East to trump with the four, leaving him with the 9 and 8 of spades to South's Queen and five. East takes the last trick for his 10th, making the contract.
If East tries to pull trump before taking all his sure tricks and getting his two diamond ruffs, he will be down at least 1 and maybe 2.