"2/1 game forcing (Two-over-one game forcing) is a bidding system in modern contract bridge in which, after a one-level opening bid, a non-jump response in a new suit at the two level commits the partnership to bidding at least game.
Game forcing auctions
The 2/1 auctions are 1♥–2♣, 1♥–2♦, 1♠–2♣, 1♠–2♦, and 1♠–2♥. Hands without an opening bid (generally 12+ high card points) are required to respond 1NT to 1♠, or to 1♥ if they lack 4 spades.
Exceptions
Some pairs don't play that 1♦–2♣ is game forcing, although some do. Also, 2/1 game forcing doesn't apply to a passed hand, or if there is an intervening call by an opponent. Some pairs play that 2/1 isn't absolutely game forcing; the pair can stop below game only when responder rebids his suit. For example, 1♥–2♣; 2♥–3♣ is treated as nonforcing by some 2/1 players. A regular partnership should discuss this possibility.
1NT Response
Because the two-level responses are stronger than in Standard American bidding, the response of 1NT to 1♥ or 1♠ opening is forcing for one round and is used (among other things) for weaker hands containing low-ranking suits. Since the 1NT response is forcing, hands with a three-card limit raise can start with 1NT and later jump-support partner. See Forcing notrump for additional details. Some pairs play a variant in which the 1NT response to 1♥ or 1♠ is semi-forcing.
Other features
Use of the 2/1 system usually implies (at least) the following additional agreements:
Five card majors, i.e. major suit openings require at least a five-card suit;
Either limit raises of major suit openings (i.e. the jump raise 1♠ – 3♠ or 1♥ – 3♥ shows a game-invitational hand with at least four-card support) or Bergen raises;
Inverted minor raises, in which a jump raise of a minor suit opening is a weak preemptive bid, while a single raise is strong and forcing for one round;
Jacoby transfers over 1NT opening;
Jacoby 2NT, showing strong support with 4 or more cards;
Fourth suit forcing and artificial.
Examples
1♠ – 2♣
2♦ – 2♠
Forcing to game, with original spade support and good club suit. This is different from standard bidding, in which such a sequence would show about 10 points, and club suit could be semi-fake.
1♠ – 2♣
2♠ – 2NT.
Forcing to game, with balanced hand and a good club suit.
1♠ – 2♣
2♦ – 3♣
Forcing, unless the partnership has agreed that this is an exception to the "2/1 rule."
1♦ – 2♣
Forcing for one round only (as in Standard American), except in the variant of 2/1 where this sequence is game forcing as well.
1♣ – 2♣
Forcing for one round; 10 points or more with at least four clubs.
1♣ – 3♣
Weak; 9 points or less—sometimes much less—with at least five clubs.
1♥ – 2♥
Weak; 6-9 points with at least 3 hearts (unless Bergen raises are in use, in which case it shows precisely 3 hearts)
1♠ – 1NT;
2♣ – 2♠
Shows a weak hand, 6-9 points, with precisely two spades. Some also use this for an extremely weak hand (0-5) with three spades.
1♠ – 1NT;
2♣ – Pass
Shows a very weak hand, perhaps 5-7 points, with at least four clubs.
1♠ – 1NT;
2♣ – 2♦
Shows a weak hand, 5-9 points, with a long diamond suit.
1♠ – 1NT;
2♦ – 2♥
Shows a weak hand, 5-9 points, with a long heart suit.
1♠ – 1NT;
2♥ – 3♣
Shows a weak hand, 5-9 points, with a long club suit.
1♠ – 1NT;
2♣ – 2NT;
Shows 10-11 points without support for spades.
1♠ – 1NT;
2♣ – 3♠
Shows 10-11 points with 3-card support for spades.
1♠ – 3♠
Shows 10-11 points with at least 4-card support for spades.
1♠ – 1NT;
2♣ – 3♥
Shows 10-11 points with a long heart suit.
1♦ – 2♥
This is a jump response, and there are different ways of handling it. In Standard American, such a "jump shift" shows a very strong hand and is unequivocally forcing. However, since such hands do not occur with great frequency, it is more common today to use such a bid to show a weak hand with a long suit, unsuitable for defense. Another possibility is to play it as a "fit-showing jump", showing 8-10 points, a decent heart suit, and good diamond support."
SOURCE: Wikipedia contributors."2/1 game forcing." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 8 Mar. 2013. Web. 30 Mar. 2013.