In matchpoint it is important to understand the posible gain or loss resulting from doubles.  Here is a prime example:
If your opponents get to a bad slam contract and they go down you will get a very good score.  The field will be scoring 650, 660 or some such score and you will have a plus 100 for a very good score.  If you double you might get an extra matchpoint.  But if the slam is a good one the field will be scoring 980 or 990 and your opponents will score 1210 or 1230.  By doubling you turned an average result into a bottom.  Worse would be to double a minor suit slam when the field is bidding six no trumps or you double a slam you know will not make and your opponents run to a makeable slam.  Now you have turned a top into a bottom.
The double of a slam usually calls of an unusual lead from your partner (A Lightner Double).

The double of a five of a major contract the was bid to make (not a sacrifice bid) or a double of four of a minor is also bad matchpoint bridge.  If you beat five hearts or five spades one trick you will get a very good score.  If your opponents bid four diamonds andmake five or go down one you will usually get a good score.

Be careful of doubling part scores when the opponents are not vulnerable and you are vulnerable. It is very difficult to defeat a part score contract enough to compensate for a vulnerable game. Even of one trick set is not enough to compensate you for a lost two of a minor contract.  Also be ready to double vulnerable opponents when your game might be in doubt.  A doubled vulnerable two trick defeat is all that is necessary to compensate for a not vulnerable game.