Responding to a Weak Two in a Minor
You're probably all familiar with using 2NT as either a feature enquiry or as Ogust in response to partner's weak two bid. These are both fine when partner's weak two is in a major, but when partner bids a weak two in a minor, there are two problems with this:
The solution to the second problem (suggested by John Haslegrave) is to switch the meanings of step one and 2NT, so 2NT is a natural forcing bid showing the suit of step one—diamonds in response to a 2 preempt, hearts in response to a 2 preempt. The point is that after a forcing change of suit opener will nearly always want to either rebid his suit or raise the new suit, so there is little value in having a lot of space in this auction—we might as well use 2NT to show this. Now step one becomes a feature enquiry.
This allows the following solution to the first problem: When we make a feature enquiry, partner can show a major suit feature with either a useful honour (Ace or King) or a reasonable side suit (Qxxx or better). We can then make another bid (usually 2NT) to enquire which of these he actually has.
That's the basic idea of the responses. There is another refinement: usually it will be better for responder to become declarer, since he has not (and will not) described his hand as well and he should have more points. Therefore, to maximise the chance of this happening, the major suit responses are reversed, so a 2 or 3 response shows a spade feature or sidesuit, and a 2 response shows a heart feature or sidesuit.
We mentioned that responder can enquire whether opener's major suit is a feature or a genuine side-suit, but didn't elaborate on how this works: if opener has been able to show his feature at the 2 level, then 2NT is the enquiry, and the responses are as follows:
After this 2NT enquiry and any response other than 3 of the minor, the major suit is agreed. If 2NT is not available (this only occurs on the auction 2—2—3), then 3 is theenquiry, and in response, 3NT denies a heart side-suit, 4 shows a 6–5 shape, 4 shows a 4=2=5=2 shape, and 4 or 4 shows a singleton or void in that suit.
Here is the full system (as far as opener's second rebid) in response to a weak 2 opener:
The response structure to 2 is very similar.