Posted 2023-January-17, 21:41
Pretend for a minute you're not playing with GIB.
In standard 2/1, if your partner opens the bidding and you have a weak hand with a long suit that you want to be trumps, you bid a forcing 1NT, then your suit at the lowest level over whatever partner does - virtually always a signoff saying you have no interest in game.
Sometimes this sign-off is at the two level - eg 1♠ - 1N - 2♣ - 2♦, weak with long diamonds.
But sometimes it has to be at the three level - eg 1♠ - 1N - 2♠ - 3♣, weak with long clubs, or 1♠ - 1N - 2♥ - 3♦, weak with long diamonds.
If you have an invitational hand with a long minor, you therefore often have no way of showing it. You can't start with a game forcing 2 over 1, and if you start with 1NT you might be stuck when bidding the suit would show a weak hand (and you're certainly not going to jump to the 4 level when not strong enough to bid game).
Therefore it's common to play invitational jump shifts, where jumping straight to the 3 level shows precisely that hand that's impossible to otherwise bid - invitational with a long minor. Opener can pass if a minimum, or bid on to game with extras. Extremely useful when playing with humans.
GIB, of course, is a bean counter and has no idea how to properly evaluate hands, so it often makes an invitational bid when it should be forcing to game (it won't game force with 11 HCP even with enough distribution to make up for it). Nothing you can do about that.