The Good-Bad 2N bid is artificial and shows a hand with the lower of the 2 possible value ranges in each case. It is nominally forcing, and nominally relays to 3C (allowing the NT bidder to pass or select a signoff location). In some cases, partner will correctly refuse to relay to 3C.
Our rules for when it applies differ somewhat from Bergen's. We play that it is on under these conditions: 1. RHO's most recent call is a suit bid at the 2 level 2. our side has made at least one bid which is not a pass 3. the bid is obviously needed to distinguish the value ranges.
A few typical examples of Good-Bad 2N (opponent's bids in parentheses)...
1H - (Pass) - 1N - (2S) - ??
3H = invitational values, natural
3C or 3D = invitational values, natural
2N = Good-Bad 2N: competitive values, with suits unspecified
(1S) - DBL - (2S) - ??
3C or 3D or 3H = invitational values
2N = Good-Bad 2N
(1C) - DBL - (1S) - Pass; (2S) - ??
3C or 3D or 3H = invitational values (opposite a pass:
this should be a very strong single-suited hand)
2N = good-bad 2N (strong single-suited hand willing to compete)
1H - (1S) - 2D - (2S) - ??
3C or 3D or 3H = game-forcing
2N = Good-Bad 2N, only invitational opposite a minimum 2D bid
Sources: Marty Bergen (Better Bidding Volume 2).
hoffman@hal.COM (John Hoffman)