Source: Andrew Gumperz Blogspot

At Matchpoints, the number of total points by which you beat another score has no value at all. Being 10 points better than the other players is the same as being 1000 points better. The focus is actions which will lead to a higher score more often, regardless of the size of total points risked.

Approximately 50% of all hands are played in a game contract, 40% in a partial and 10% in a slam. At IMPs, the slam hands and game hands are disproportionately important because of the large bonuses involved. For example, about 20% of IMPs swing on slam hands, even though they only occur 10% of the time. Consequently there is a high premium on accurate slam and choice of game bidding. At Matchpoints the same number of matchpoints swing on every hand regardless of level. Thus game and partscore bidding is the focus because of their frequency.

At IMPs, the plus score is king. Both declarer play and defense are usually all about guaranteeing the plus score. At matchpoints the goal is to optimize your score. Your goal varies from hand to hand. sometimes you must minimize the loss by holding down overtricks. Sometimes you must risk your contract to make an overtrick. Sometimes, you must score a penalty large enough to outscore the field. Sometimes you must simply guarantee your plus score. Sometimes you must hold your contract to down one instead of risking a larger negative score in an attempt to make.

Overtricks and undertricks
1. Overtricks and undertricks in all contracts matter. Unless you are certain that your contract is exceptionally good, you will normally play to take maximum tricks.
2. When opponents declare and they likely have full values (2NT-P-3NT-all pass) defend passively. Dont lead from AJx or KJx because the only chance to beat the contract is catching partner with 5-cards in this suit. you will blow 10 overtricks for each miracle defeat of 3NT you achieve.

3. Even against an auction like 1NT-3NT, a safe lead like K from KQJ is often better than an risky lead from a broken 4- or 5-card suit because of the chance to hold down the overtricks.

4. Holding down an overtrick can be just as valuable as bidding a slam. It is worth the risk of occasionally going for a number. Therefore, when you can only stand a lead of one suit, take risks to overcall that suit or double an artificial call for a lead.
Field Contracts
1. The field contract is very important to matchpoint thinking. In the US, if you play weak NTs, you will often play contracts from a different side than the field. Every time you do, you must think about whether your reversal has put you in a higher scoring, lower scoring or equal scoring position. Depending on what you conclude, you may adjust your play approach.

2. When you have reached a contract that rates to matchpoint poorly, grasp at any chance to improve your score. For example, suppose you bid 3NT with an 8-card major suit fit in a try for a top. Unfortunately you judged badly and the 4M players have an easy path to ten tricks while you have only 9 top tricks in 3NT. However, there is a 25% line which will either make an overtrick or cause you to go -1. Playing safe will earn you zero since +400 loses to +420. The 25% line will probably cost you nothing when it fails but wins 11 or 12 matchpoints when it works.

3. The reverse is also true. If you have gotten to a cold 4-spades in a 4-3 fit that will outscore both 3NT and 5m, take no risks in the play. if +620 and +650 will both get you 12 matchpoints, why risk -100? Suppose you are defending a doubled, vul contract when you could have made an NV game. You can see a sure +500 set. Do not try for +800 if you risk collecting only +200. +500 will be a near top. +800 offers you no reward when it is right and could cost you a full board when it is wrong.
Vulnerability

1. Vulnerability tilts many close competitive decisions. A hand that is a clear double when both sides a are vulnerable, may be an equally clear bid over the opponents when both sides are NV.
2. Double aggressively for a -1 set when oppos are vul. +200 is a top.
3. Overbid for a -1/-2 result when you are NV. -50 or -100 is better than -110 or -140.
4. NV on NV is the best time to compete for the partscore. You can get away with outrageous actions to balance or compete for a partial. For example, when the opponents have stopped in 1NT, NV on NV, balance on almost anything. Defending 1NT is very likely to be a poor matchpoint score. If you bid you will push them out of their high scoring contract 75% of the time or more. NV on vul is also a good time to compete for the partial, especially via a takeout double (because partner may be able to pass it to earn +200). You and your partner should discuss how the vulnerability affects how frequently you bid on these auctions. For example, after 1NT-P-P-? at NV on NV, it will be a winning action to double light, if you can rely on partner to remove it unless he holds a hammer. However, if partner passes on his flat 8 count you may be unhappy. Make sure you and partner are on the same wave length that your double may not show great strength, it is simply an attempt to push them out of their comfortable resting spot.
5. Vul on Vul is the worst time to keep for the partial. Prefer to defend unless balancing or competing is very clear. Vul against NV is slightly better, but it is still a situation where you should be careful.
Preemption
1. Preempt aggressively when NV
2. When oppos can make a game, take a pushy sac when you are NV
Penalizing
1. When white on red and holding approximately game values, double for penalty aggressively when the opponents interfere. +500 on defense beats an NV game.
2. When oppos are NV and they take a likely cheap sacrifice, compete to the 5-level aggressively. If the field misses the sac, you have a zero by defending. try to get back to average by over bidding. At IMPs you dont mind collecting +300 against a game. At matchpoints, that is a likely zero. You are better off bidding on to 5 and hoping for +450.

Defensive bidding
1. Take risks to make bids that will disrupt the opponents.

2. When you know your side rates to lose the auction, lead direction becomes the bidding priority.

3. When the opponents will have a game bid extremely aggressively.