NMF primarily addresses how to find an 8+ card fit when responder has 5 cards in his first bid major suit and declarer has 3 card support. Eight card fits are important since they will out perform no trump contracts a majority of the time. Let’s start with that simple fit finding objective.
The bidding goes 1d/1s/1NT/2c. When we play NMF the bid of two clubs, (a new minor) is forcing for one round. Opener cannot pass, he must bid. Openers options will be described later.
What are the clues that it is NMF? (i) partner has opened (generally with a minor) (ii) responder has bid a suit at the one level (in this case spades) (iii) opener has rebid 1NT showing a balanced 12-14 hcps and responder now bids an unbid minor.
There is one NMF sequence where partner will not have opened a minor – 1h/1s/1NT/2m. As long as the rebid is 1NT, the bid of the minor is still NMF. Since a minor has not yet been bid, it’s your choice. Pick your better minor. While in NMF the bid minor does not necessarily show a suit, in this case you minor implies a stopper since you had a choice between the minors. If there is no major suit fit, bidding the better minor may be helpful information in finding a no trump contract.
Now let’s jump to responder’s side. Responder cannot use NMF unless he has at least 11 hcps since opener has limited his hand to 12-14 by rebidding 1NT. There is no maximum on responder’s NMF bid. In the vast majority of cases responder has 5 cards in his first bid major and wants to find out if opener has 3 card support. Yet, like its first cousin, 4th suit forcing, NMF has great flexibility and in end result is nothing more than a request for opener to further define his hand
In the sequence 1d/1s/1NT where responders bid is one spade, it is possible that responder has a hand like KQxxx, KQxx, Ax, xx. In this specific auction responder is interested in whether opener has a three card spade suit or a 4 card heart suit that opener could not show since responder’s first bid was 1 spade. Let’s now give opener Kxx, Axxx, KQxx, Jx. Does opener show his 4 card heart suit or does he show the 3 card spade suit? There are differences of opinion about the priorities. Ignoring all conflicting opinion, I play that opener’s first priority is to show the unbid 4 card heart suit, and then by rebid show the 3 card spade support if responder indicates no fit for hearts. Note that this issue only arises when responders first bid is 1 spade.
If opener does not have a holding to make a positive major suit response to responder’s NMF bid, then opener shows the hcp range of the hand as a minimum (12-13) by bidding 2NT or a maximum (14 hcps) by bidding 3NT.
If opener does have a major suit fit for responder, the same principle of showing the range of the hand applies. If you have a fit and 12-13 hcps, show the fit on the 2 level (2 hearts or 2 spades), and if you have a maximum of 14 hcps show the fit on the 3 level (3 hearts or 3 spades).
What if the bidding goes 1d/1s/1NT and you hold Kxxx, xx, x, QJTxxx. This hand will not play well in no trump and you really want to play the hand with clubs as trump at the cheapest level. Two clubs would be NMF, so with this hand you bid 3 clubs. This is a clear signal to opener to stop bidding and put his hand on the table.
NMF is also “on” when opener rebids 2NT. So 1c/1s/2NT (18-19)/3d is NMF. If responder has already made a 1 level response, he has enough to use NMF. Partner now bids 3 of a major to show 4 hearts or 3 spades and 3NT to show neither. If responder finds a major fit, he bids 4 of the major and if not passes 3NT.
The important thing about NMF is that when opener has a minimum hand he makes a minimum response at the 2 level (up to and including 2NT) and when he has a maximum he makes a jump response at the 3 level (up to and including 3NT)..
There are a number of optional more sophisticated treatments that can be used in connection with NMF, but this represents all the basics. Don’t forget that the NMF and its responses are alertable.