An opening bid of 1NT on 12-14 points is very descriptive to partner but also highly pre-emptive to the opposition. Following are a few guidelines on how to compete against a 1NT opening.
With a balanced kind of hand and up to 14 points it is best to pass and defend.
With 15+ points make a penalty double.
The time to compete is on more unbalanced hands but playing strength and suit quality is more important than high card points. Thus KQxxx x KJ10xx xx is better than Jxxxx Kx KJ10 AQx as despite fewer points, the hand has better distribution and concentration of values in the long suits.
Intervening is much more attractive based on shape rather than points and, as a result, many defences to 1NT have been developed over the years to be able to offer two suits rather than one.
There are different schools of thought as to when to compete against 1NT, some think you should do so at every opportunity while others are more discerning. I belong to the latter school but whichever you decide, the fact remains there is always an element of risk, especially as second bidder as you don't know where the rest of the high card points are. If the majority are with partner, competing will probably be right but if they are with the opposition, you may have landed yourself in trouble.
Looking at the above deal, after South opens 1NT, West has to decide whether or not to compete. If you are playing a 'natural' defence, you would have to bid 2♠ and that would not be a good contract at all and if East tries to rescue into hearts, things only get worse!
However, if you are over cautious you will miss opportunities. If you were to transpose the North and East hands, playing in spades would be excellent and you could make as many as ten tricks.
THE ASPTRO CONVENTION
Of the multitude of defences, Asptro is a good and flexible defence mechanism to an opening 1NT. In its most simple form there are two artificial overcalls and two artifical responses.
An overcall of 2♣ promises hearts and another suit. An overcall of 2♦ promises spades and another suit. I would recommend you anchor to a five card major so a bid of 2♣ therefore promises at least five hearts and at least four of another suit.
Overcalls of 2♥, 2♠, 3♣ and 3♦ are all natural and should really be based on a 6 card suit.
As mentioned above, playing strength is more important than high card points but you would not have more than 14 as otherwise you would have doubled though you could have as few as 9 or even 8 if your hand was distributional, eg KQxxxx x x Kxxxx. In principle however partner should expect you to be nearer to 11-14 points and at least 5-4 for an overcall of 2♣ or 2♦.
In many cases, partner will have a good idea where to play, game or part-score and with three or more of the major you have shown, he can bid it at the 2, 3 or 4 level depending on his hand.
There are two artificial bids on the other side of the table. In response to an overcall of 2♦ (showing spades and another), partner may bid 2♥. This does not necessarily mean he has a heart suit but simply denies having three or more spades. Similarly 2♦ after an overcall of 2♣ denies having three or more hearts. These bids are normally made on weak hands with no fit and no particular aspiration to go much further.
The other artifical bid is 2NT. This is conventional and asking partner to bid his other suit.
The above hand is a good example of how this works. West bids 2♦, nothing to do with diamonds but showing at least 5-4 in spades and another suit. Clearly East does not want to play in spades but is happy to play in anything else. He therefore bids 2NT which asks partner to bid his other suit. It happens to be diamonds and 3♦ is a much more comfortable final contract than 2♠ (doubled) would have been.
The advantage of using a convention such as Asptro is that it enables you to show partner you have two playable suits and unless you have a 6 card suit (which you can bid naturally) or a two suited hand which you can show by bidding 2♣ or 2♦, it is often not worth intervening in the first place. On the above hand, showing a single suit, rather than making partner aware you have two, is dangerous. Being able to show two suits enables you to play in a good contract instead of a poor one.