Positive Quiddity: Ruffing

In trick-taking games, to ruff means to play a trump card to a trick (other than when trumps were led). According to the rules of most games, a player must have no cards left in the suit led in order to ruff. Since the other players are constrained to follow suit if they can, even a low trump can win a trick. In some games, like Pinochle and Preferans, the player who cannot follow suit is required to ruff. In others, like Bridge and Whist, he may instead discard (play any card in any other suit). Normally, ruffing will win a trick. But it is also possible that a subsequent player will overruff (play a higher trump). This is not always a bad thing—see uppercut below.

Usage of the word "ruff" vs. "trump"

"Ruff" is normally a verb, meaning "to play a trump card when a non-trump suit was led". "To trump" can be used as a synonym of "to ruff", but "ruff" is normally preferred, for clarity. As a noun, "ruff" and "trump" are completely different -- "a ruff" means only "an instance of ruffing", while "(a) trump" means only "the suit that outranks all other suits", or "a card in this suit". Hence:

• One can "give a ruff" to partner but not "give a trump".
• "Hearts are trumps" but not "Hearts are ruffs".
• "Cross ruffing" and "cross trumping" are both correct, but "cross ruffing" is preferred.
• "Ruff and discard" is common usage but "trump and discard" is not.

Ruffing and cross-ruffing in partnership games

A player gives a ruff by leading a card of a suit in which partner is void, enabling the trick to be taken with a trump card. Partner will then attempt to get the lead back into the original hand, by leading a certain suit, so that the process can be repeated.

• A crossruff is a play where tricks are made by taking alternate ruffs in each hand. In order to use a crossruff, each player in the partnership must have shortness in a non-trump suit, accompanied with appropriate length in the opposite hand. Also, each partner must be short in the suit that his partner is long in. It is preferable that both players have an equal number of cards in the trump suit, otherwise a regular ruff is usually more effective, as it has the added benefit of establishing the trump suit.
• The mechanics of the crossruff are simple but the effect can be dramatic as this extreme example, from a bridge hand, demonstrates:

West holds                              East holds

     Spades AJ8543                              Spades Q109762

     Hearts void                                    Hearts void

     Diamonds void                              Diamonds 9876543

     Clubs 9876543                               Clubs void

West plays the grand slam of 7♠ despite having only 7 high card points. The declarer can draw the outstanding trump king, ruff the diamonds in dummy, going back to the hand by club ruffs. Unless both minor suits are divided 6-0, one of the minor suits will ultimately become high and provide the missing two tricks. In summary, the declarer took one trick by leading a high card (the ace of trumps) and 10 tricks by cross-ruffing; the remaining two tricks came as result of long suit establishment.

• However, there are risks with crossruffing: when the opponents also run out of cards in the suit(s) being ruffed, they can overruff (play a higher trump card); also this play may leave the trump suit unestablished so that opponents may be able to steal back a trick or two using their trumps later. Crossruffing is therefore just one possible strategy for taking extra tricks, others are establishing the trump suit, traditional ruffing and finessing.
• Bridge considerations: the additional information given by the existence of the "dummy" in bridge produces many opportunities for ruffing and cross-ruffing
• It is often important to cash side-suit winners before commencing a cross-ruff, otherwise the opponents may discard in the side-suit, allowing them to trump the winner later.
• In some cases, it is effective to cross-ruff after drawing the opponents' trumps, when this can be done with trumps remaining in both hands.
• In other cases, it is effective to cross-ruff only until one opponent becomes likely to be void in a particular suit, and revert to drawing trumps thereafter.
• The basic defense against crossruff is simple: lead trumps whenever possible, removing trumps from both opponent's hands. In bridge this may mean that, the defense must lead trumps from the opening lead in order to prevail. Thus, it is important to recognize the situations when a trump opening lead is called for – usually, they arise when both declarer and dummy have bid other suits but found the trump fit in the third one.

http://en.wikipedia.org/wiki/Ruff_(cards)#Ruffing_and_cross-ruffing_in_partnership_games