To expand on that a bit, modern bidding in competition is all about
the last guess. Let's say you are sitting at the table and are watching the auction (1
♦)-1
♠-(P)-?, and it is your call. There are many different hands that will want to support partner. Some hands will be weak and shapely (e.g.
♠Axxxx,
♥xxx,
♦-,
♣xxxxx) and will want to give a preemptive raise to shut the opponents out. Others may be strong and flat, but with support (
♠Kxx,
♥AKx,
♦Qxx,
♣Jxxx). Other hands may have both strength
and shape, and in modern aggressive bidding people even bid with
neither points nor shape (
♠KTx,
♥xxxx,
♦xx,
♣xxxx is an acceptable 2
♠ bid these days).
The old school (and not strictly wrong, but certainly incomplete) theory is that you have to warn partner when you are raising on a weak hand, so that partner doesn't propel us to some hopeless game when we were merely bidding 'tactically'. This is why in standard systems there are usually two ways to raise to 2
♠ - in this case 2
♦ and 2
♠. The cue says "partner, I have a strong raise to 2
♠" while the other says "I am bidding for obstructive purposes".
The modern approach to competitive bidding tries to not only prevent getting to bad games, but also tries to prepare partner for the next round of interference by the opponents. There are many different ways to try to assist in this, but the simplest one is based on high card points and the law of total tricks. The idea is that if we tell partner of the exact degree of the fit we have, partner will know whether it is 'safe' to raise. And if we tell partner how many points we have, partner will know if we are being stolen from. For example, assume the bidding starts (1
♦)-1
♠-(P)-2
♦; (3
♣)-?. If advancer has a fourth spade, it is often correct to compete to 3
♠ based on the law of total tricks. But if advancer has a flat hand, anything from pass to penalty double to 3NT may be best. The problem is even greater if the bidding is pushed to higher levels: (1
♦)-1
♠-(2
♦)-3
♦*; (5
♦)-? and now what? The modern approach to competitive bidding is to give advancer multiple different ways to raise partner, clarifying what sort of raise they have as early as possible. This way overcaller has an easier guess over interference, without compromising game/slam decisions too much.
Over the example auction (1
♦)-1
♠-(P)-? a relatively common and relatively simple scheme is:
- 2♦: point raise (say 9+ points with 8 or fewer losing tricks) with exactly 3 spades
- 2♠: destructive 3-card raise, around 4-8 points (usually)
- 2NT: point raise (as before) with 4 or more spades and no good 5-card side suit
- 3♣: fitbid (5 or more decent clubs, 4 or more spades, at least 8 points or so by necessity due to required club strength)
- 3♦: mixed raise (4 or more spades, around 6-9 points)
- 3♥: fitbid (as above)
- 3♠: preemptive raise (0-5 points, 4 spades, or sometimes 5 spades with an otherwise flat hand)
- 4♣: a splinter (10-14 points, 0-1 clubs, 4(+) spades) or fitbid (the same but 5(+) clubs instead) depending on agreements
- 4♦: a splinter (as above)
- 4♠: a preemptive raise (same as 3♠ but with an extra spade)
This is far from perfect, but by giving ourselves 10(!) ways to raise partner they will hopefully have less trouble deciding what to do on the next round. For example, in the auction in the first post, by excluding the point raises with at least four spades from the 2
♦ cuebid overcaller can be assured that they don't have to take action on the 3-level with a boring minimum - the 'law protection' ends at 2
♠, and without extras there is no game try.
As a final note, maybe this should be the opening sentence, I should add that it is/was popular to also start with a cuebid on very strong hands without support. Generally partnerships that allow this hope that the strong hands will land on their feet no matter what, by clarifying the hand type on the second round. And if partner goes crazy and jumps to a high spade contract immediately, usually they will have extra spades and your extra values will make up for the lack of spade support. This doesn't change any of the tactical considerations of bidding, except that every once in a while you will have to go "I had the strong hand instead of support" when tabling dummy.