The Weak Two Opening Bid (2D or 2H or 2S) is a preemptive bid. It shows a 6-card suit, 2 honours in that suit, no other 4-card Major and a weak hand (6-10HCP).
Weak Twos have the advantage of preempting the opposition and making it difficult for them to find a contract.
When using this bid, have at least two honours in your long suit (count the 10 as an honour if you have the Ace).
Don't make this bid if you are vulnerable because going down would be expensive.
Don't make this bid if you have a four card major suit. Partner may also have 4 cards in that major and you are likely to miss the fit.
The Opener at 2 level must not bid again unless asked by the Responder. You have described your hand.
Don‘t make this bid in 4th seat after 3 passes unless you have 10-13HCP. Remember you need 19-24 HCP to make part game.
In general, bid to the level of the fit to make it difficult for Opposition to find their likely game contract. Except, with 4333 shape, raise only to one less than the level of the fit. This should be enough to cause problems for Opposition.
With no fit, but you are long and strong (16+) in another suit, you can make a non forcing raise. If Opener has a maximum hand, he may show another suit and you may then decide on a NT contract. If Opener has 3 card support in your suit and a maximum hand he should bid to game.
With no fit and an intermediate hand, it may be best to pass. Partner is unlikely to have much support for your suit and you probably only have enough points between you for 8 tricks.
With a strong hand (16+), we use the 2NT enquiry to ask partner to describe his hand. If partner is minimum (in HCP) he signs off by rebidding his suit. If he is maximum he shows his best feature in another suit (A or K usually) or bids 3NT if almost all top honours in his suit but little else. If he shows a feature, you can then decide if you should try for game or not.
If Opposition intervene by doubling the Weak Two Opening and you have an intermediate hand and no support for partner’s suit consider leaving the double in. You may be able to just about make 8 tricks but no more. If you have support for partners suit consider raising it to further pre-empt opposition.
If Opposition intervene by bidding a suit after a Weak Two Opening and you have their suit and an intermediate or better hand and no support for partner’s suit consider doubling them. This is for penalties.
To defend against a Weak Two Opening, bid more or less as you would over the opening bid of one of a suit.
With two stoppers in the Opener’s suit and a strong hand (16+), bid 2NT.
With a strong hand but no stoppers and with at least 3 cards in the other suits, bid a takeout double.
With opening points and at least three cards in each of the unbid suits, bid a takeout double.
With opening points or more and a long suit, bid the long suit.
With a rule of 20 hand, bid the longest suit.