Balancing is probably the most important tool in matchpoint bridge.   It is best defined as 'reopening the bidding by bidding part of your partner's values when the opponents have stopped bidding at a low level.'

Knowing when to balance (Called protecting in England) is very important.  If your opponents have not found a fit you should rarely balance.  If they do not have a fit you are not likely to have one. By balancing when they have not found a fit you may allow them to find one or, worse yet, you may get doubled and suffer a 200+ penalty.

If they have found a fit or stopped at one of a suit it is important balance.  The following two auctions are 'must balance' auctions:  1 Pass 2 Pass;   Pass ?    or    1 Pass 1 Pass;   2 Pass Pass ?.  There are three reasons to not balance after a 'must balance': Four cards in their suit and a weak hand, a complete bust hand, or a weak hand and weak opponents who might bounce into game.

One more situation is important:  If opening bidder rebids 1NT and you have three cards in responders suit you should usually not balance.VThe opener may decide to show three card support for responders major suit and find an excellent fit with a good split.  If you hold none or one card in that suit it is often a good idea to balance.