Source: Mr Bridge
Nobody is vulnerable and RHO deals and opens 3
. What do you do with this?
You have a hand on which you would happily open 3
non-vulnerable but you should not overcall 3 because they opened with a weak bid. Say you bid 3 and LHO raises to 4, which partner doubles (cards, not strict penalties) —what now? Have a look at the full deal:
If you bid 4 at this point, you will be doubled and lose six tricks (two diamonds and a ruff, two spades and one club) costing 500. If you pass, you will concede 4+1 for minus 690. Yet partner surely was entitled to double because you overcalled at the three level and he had 10 points with a singleton in your suit. The point is that, when the opponents make a weak bid, then your side should bid constructively hoping to reach game (or sometimes a slam).
Do not make the mistake of bidding on a weak hand over their weak bid. (I remember learning: ‘don’t pre-empt over a pre-empt’ ?) Another example:
RHO opens 2 (weak) at game all. Do not make the mistake of overcalling 3. If you do, this could be the layout.
Partner will try 3NT and with some confidence! West will double and you will go for 500 on a soft defence and possibly 800 or 1100, yet partner could hardly pass 3. Remember it is important when bidding over a weak bid that partner can trust that you will not be weak too. The opposite is true when bidding over a strong bid. You do not want to bid on a good hand and tell them where the cards are or concede a penalty when you had enough to beat them. Now your aim is not to get to game but to obstruct the opponents’ bidding or, occasionally, to find a good sacrifice.
For example: 
RHO opens 2
(game force or 23-24 balanced). Your side is at favourable vulnerability. Do not be afraid to get in their way. Try overcalling 3 ( ‘weak over strong’). Maybe the full deal is something like:
Opponents are cold for 7 or 7
, or 7NT. Good luck to them in finding it; they will struggle even to reach the small slam after your 3 bid. If you pass, West can respond 2 and over East’s 2NT rebid can bid 3 showing a slam try with both minors (a 3 transfer would show spades); now they are well on their way.
Summary
The point is to bid destructively when they make a strong bid and to bid constructively when they make a weak bid. In other words: Bid Weak Over Strong and Strong Over Weak!