Source: ACBL
At IMP scoring, you focus on beating the contract. You may need to make assumptions that will leave you with only one possible line of defense. At matchpoints, your score depends on how many of your competitors you beat — by any margin.
Defense is more taxing because your goal may be to hold declarer to one overtrick or to try for down two and plus 200 against a partscore. You must often assess the contract as «normal» or «abnormal,» consider whether the defense has started well or badly, or judge whether your side would have done well at a contract of its own. 
West leads the 
K. Declarer takes the ace and leads a trump to dummy’s jack and East’s ace. At IMPs, East would return the 
2. To beat 4
he needs at least one diamond trick and must hope South has a hand such as:
A2 KQ109842 K7 
K6. (South could make his game by taking a quick spade discard on the clubs, but that play wouldn’t be clear.)
At matchpoints, East must consider returning a spade to take his side’s spade trick or tricks. At least one spade should cash since West might have overcalled despite the vulnerability with K–Q–J–x–x and another high card. The contract appears to be normal: Every North–South will be in the same spot.
If East leads a diamond and finds South with: A2 KQ10982 A7 K65 East–West will be minus 680 for a zero.
South misjudges and bids 5, thinking North’s hand may be stronger and East’s weaker.
West doubles and leads his singleton heart, and East takes the ace and returns a non-committal 5 for West to ruff. At IMPs, West might try for plus 800 by underleading his A to get another heart ruff. At matchpoints, defenders often set a trick target that will obtain a good result, and West should cash the A, assuring plus 500 and beating East–Wests who are plus 480 at spades. (Not many East–Wests will be plus 750 at 5 doubled.) 
West leads the 3: K, A, 5. South’s 2 rebid promised a six-card or longer suit, so he has at least six heart tricks and three spades (he has the A for his opening bid).
South appears to have Q–x–x in diamonds: He wouldn’t play the king from dummy if he had the jack, but West would have led the queen from Q–J–x–x. If South also has the Q (likely), East must make sure he doesn’t make six. East should cash the A at the second trick. If instead he returns a diamond, South wins, ruffs a diamond in dummy and runs his trumps, squeezing East in spades and clubs for 12 tricks.