You really need a bid in your system to describe a strong raise for partner’s major opening. Suppose partner opens 1♠, and you hold: ♠K1084 ♥AJ7 ♦AQJ6 ♣83
If you play limit raises (10-12 pts), you can’t afford to bid 3♠ as partner might pass. With 15hcp + 1 for the doubleton, you need to be in at least game.
In keeping with a raise to 2♠ showing 6-9 pts, and a raise to 3♠ showing 10 – 12 pts, the most natural progression would be to raise to game with 13+. But we would lose 4♠ as a pre-emptive raise (5 trumps and 6-9 pts), and would not leave room to explore for slam without getting past game.
Jacoby 2NT can be used for this purpose.
• A response of 2NT to an opening 1♥ or 1♠ shows a game-forcing raise of the major and requests more description of opener’s hand. This 2NT response is not natural (i.e. a balanced hand with a desire to play in no trumps).
• 13+ total points
• Four–card support for the 1♥ or 1♠. With enough for game but only three trumps, make a delayed game raise (bid their own suit, and then raise opener’s).
Opener’s Rebid after Jacoby 2NT
Opener now shows shape regardless of strength – either shortness in a side suit or length in a side suit
• A new suit at the 3-level shows shortness, a singleton or void (eg 1♥ p 2NT p 3♣/♦/♠ = a singleton or void in the suit bid)
e.g. ♠7 ♥QJ1053 ♦AQJ10 ♣KJ3 You open 1♥, partner bids Jacoby 2NT and now you'd show your spade shortage by bidding 3♠. Partner will now decide whether to go beyond game.
• A jump to the 4- level in a new suit shows at least five cards in that suit, and a very good suit (eg 1♥ p 2NT p 4♣/♦/♠ = a good second suit) e.g. ♠K2 ♥QJ1053 ♦AQJ108 ♣4 You open 1♥, partner bids Jacoby 2NT, and your next bid would be 4♦.
With no singleton or void or second suit, opener shows the strength of the hand
• With minimum strength, (12-14), jump to game. It’s the most discouraging bid opener can make in terms of slam interest (eg 1♥ p 2NT p 4♥) ♠82 ♥QJT53 ♦AQT5 ♣K6. you open 1♥, partner bids Jacoby 2NT, and now you show the minimum with no shortage by jumping straight to game, 4♥
• With maximum strength, (18+), rebid trumps at the three level ♠K2 ♥QJT53 ♦AKT3 ♣AQ. You open 1♥, partner bids Jacoby 2NT, and you would show your good hand without a shortage by bidding 3♥.
After opener has described the hand, responder can decide whether to pursue slam possibilities or settle for game in the major.