I think the subject of overcalling is interesting. Let's start with my reflections and opinions of 2-level overcalls after opposing 1M-opening. This is an area I've been meaning to write about a long time but never gotten around to. As this issue stirred up some attention in my last post(s), let's move on and destroy any credibility I may have left ;-)
There seem to be a consensus among many experts I've talked to that you should have a good suit, preferably 6 or more, and not be shaded when overcalling at the 2-level in a lower ranking suit. This is a view that I don't share at all.
The main reasons for those requirements are that it makes constructive bidding easier (say the next guy jumps to game and now you can compete to the 5-level much more comfortable with marginal hands) and to lower the risk of getting nailed for a number. If you have HHxxxx, this would make it less likely of length/strength behind and you'd still take some trump tricks of your own.
I find it useful to overcall almost any hand with a 5-card suit IF the hand has above average strength, say a control-rich 12/13+ hcp and that a weak suit may be a better proposition than a good one! Doesn't this sound strange? Why would I think this?
Here are my general thought/views and reflections on the 2-level overcall and weak vs good suits:
1) It's very useful to get into the auction. See as many 'flops' as possible. This is nothing new at the 1-level, but it may be extended to a certain degree to the 2-level as well. The chance of finding a big fit should be exploited.
2) When you have a weak 5-card suit, the chance of finding big support (length & strength) with partner is greatly enhanced, i.e better chance of gaining a high-level swing, compared to then you have a strong 6-card suit. That's the 'suit quality paradox', that a weak suit is frequently better than a good one! See more below.
3) With a weak 5-card suit, you have a better chance of getting working points in dummy when catching a raise. Why? Say you give partner 6-9 points and a 3-level raise. If our suit is weak, then partner is more likely to have honours there (as more honours out of the total is 'taken' in other suits by us/opener when we have 12+ hcp with majority outside overcalled suit) pulling offensive weight. If our suit is strong, then partner can have at most one honour there and the other, say two honours, sit in other suits, with an opening hand 'over' then making it rather likely that at least one, maybe both honours being more or less worthless (maybe slight exaggeration, but you get the point).
4) If weak suit then more strength located in sidesuits and those honours are well positioned behind opener. Better trick-taking potential when hitting the flop (dummy) and we declare.
5) If weak suit and we get caught (p-p-X), partner should adopt a more active strategy of running in this style (i.e. permitting more free-wheeling overcall) and then most of our strength will be 'working' in the alternative contract. If I have Ax/Kx/Kxxx/KTxxx and overcall 2C over 1M, then after a reopening double and a runout by partner, we have 10/13 working points in that contract. Compare to Ax/xx/KJx/KQTxxx with 8/13 and worse shape.
6) Opponents more likely to misjudge the correct level. If I have a weak suit, then they are more likely to hold some honour strength there and may devalue that holding, more than is called for. Say opener has Kx of diamonds and started with 1S. That K of diamonds is more likely to be paper waste vs a constructive typical 6-card 2D-overcall than vs a 'free-wheeling' 5-card approach 2D.
7) The perception of the risk of getting nailed is disproportionate to the actual occurence ('selective memory', we remember those better). Ok, very subjective, but I've lived it and stand by that statement.
8) Getting into the auction early have so many ways that the opp's may misjudge that they wouldn't have otherwise. The nuisance value of 2m - 3m (raise) is a real one.
9) The risk of partner's lead in our weak suit turning sour isn't very big. On game level, they are most likely to end up in opened major, then I'm on lead, or in 3NT with partner on lead and then I'd prefer my long suit led with my sidesuit honours as potential entries to establish a cash my winners. Should they reach a contract after a negative double then I'm also on lead.
All this said I don't overcall nearly as much as I used to 10-15 years ago. Busy isn't always better; I'm much more selective.
Moving on, when considering overcalls with sub-par strength (i.e. less than an opening bid) in general, meaning both 1 and 2-level overcalls:
1) The key issue on marginal (for me) decisions. comes down to possesion of a [any] short suit or not. I think that factor is the 'biggie' when it comes to the number of favorable outcomes of an overcall.
2) The second factor is holding in enemy suit (but do see previous post). Strength there on an already sub-par hcp hand is a potential big minus.
3) The control-ratio. This means the number of controls (aces = 2, kings = 1) you have in relation to the statistical expectancy for the number of hcp you have. Optimum would of course be an ace and a king and the worst would be a bunch of 'quacks'.
Let's end with a look at a real-life example of a weak suit overcall, a hand from the training weekend in Holland about a year ago against the Dutch Team Orange. You have:
White vs red, partner passes as dealer and your RHO (that would be me) opens 1D (11-13 bal/5M332 or 11-15 unbal with 4-card M). What's your call?
I'd expect a large majority, but I could be wrong of course, to pass and back in with double over expected spade bids if the level isn't to high. Jan Jansma, Hollands best player in my opinion, overcalled 2C and scored up a game minutes later.
(Yes, we open 1D on those. See postings about this from last spring.)
The board was played 4 times (2 practise matches) and all reached 5C. But, at all other tables the opening bid was 1S and it was easier. Had Jan passed, I think their chance would have been gone. Would you bid 5C on that North hand after 1D-1S; 2S (X) 4S ? That's easier when the whole hand is on display. What if partner is 1-4-5-3 ?
Was the overcall wreckless or did good bridge pay off? I think he was justly rewarded.