Source: Gold Coast Congress Boletin 2
I’m guessing the majority of partnerships play weak twos and weak-jump overcalls, without specifically discussing what the constraints on a jump overcall are. If you believe, as I do, that vulnerability is a relevant factor in the equation, not to mention level and position, then it might help to consider the following suggestions.
1. While pre-emptive jumps make sense, when the opponents have stopped bidding, weak jumps are unnecessary. Hence, at your first turn to speak jumps in balancing seat should be intermediate. With a weak hand and a long suit passing rates to be right; with moderate values – say 8-11 – make a simple overcall. A jump should be a decent suit and say 12-16 points. Equally, an opening bid of a weak-two or a three-level pre-empt in fourth seat might have a range of 8-12.
2. Against a pre-empt you need values to bid. Never pre-empt against a pre-empt. That being so, best is to use a three-level jump overcall of a pre-empt as strong – say 15-19 and a good six or seven-card suit1.
3. When vulnerable at teams, and especially at unfavourable vulnerability at any form of scoring, use jump overcalls at the two-level as 9-13 or so, not a really weak hand; and at the three-level use jumps as intermediate. The point is that if you do hold a weak jump you won’t want to make it…will you?
4. At any form of scoring, and whatever the range for the bid, use 2NT in response to a weak jump overcall as an artificial enquiry – just as you would do if partner had opened with a weak two opening. (If you are playing sound jump overcalls maybe use 2NT as asking for shortage rather than features). Incidentally if you play a weak-two in diamonds guarantees a decent suit but may have a four-card major, consider catering for that by allowing your responses to show this. 2
5. New suits in response to opening weak-twos are forcing by an unpassed hand, fit showing and leaddirecting by a passed hand. All jumps in new suits are fit-showing too (or show splinters3 in the opponents’ suits). But see the next point!
6. Play McCabe, where new suits in response to a weak two from partner and (specifically) a double from the next hand are fit-showing and lead-directing. Fit jumps also apply. Equally, after the opponents make a negative double of a pre-emptive jump, new suits should show tolerance for partner and be lead-directing. These could be described as “fit non-jumps”, to borrow terminology from Robson and Segal’s excellent book on partnership bidding.
7. After a weak two from your partner and an overcall you are awkwardly placed if you want to make a limit raise – since a raise of partner’s suit is pre-emptive not constructive, and new suits must be played as non-forcing since you will almost never have a good hand here. One way to combine an obstructive and constructive raise is to use 2NT as a relay still, even in competition. But if the intervention is at the threelevel that won’t work. I suggest you subvert a ‘natural’ call to make it show the limit raise in competition:
8. Use the suit one below partner’s suit as an artificial constructive raise for him. With the following hand 
Q-6-4 
A-3-2 
A-K-J-7-6 
6-4 after 2:[3§] you should bid 3. After 2:[3] double to show this hand – an extension of the ‘maximal double’ theory.
9. Jump cue-bids may depend somewhat on your opponents’ methods, and even in which country you are playing! Against a pair playing four-card majors, and thus where the minor-suits tend to be natural, you might consider using all double-jumps as stopper asking, suggesting a solid suit of your own. Conversely where your opponents play five-card majors, or loose minors (meaning they open a club with all balanced hands outside the no-trump range), a double jump in a minor might crop up more often as natural and preemptive. Incidentally – simple overcalls in sandwich seat4 of either LHO’s suit or RHO’s suit might as well be played as natural not two-suited. There are so many ways to show two-suiters why not cater for another hand-type?
10. Against a short club, consider using an overcall of 2 as natural. Now maybe it is better to play your 2 overcall as Michaels, not weak with diamonds? This is standard French (I’m not sure if this will encourage you to adopt it or not…). Bear in mind that if your opponents play the equivalent of a Polish Club, where a One Diamond opening shows a real suit in an unbalanced hand, then one club is ‘short’ far more often than in a run of the mill five-card major system or even in a system where you would open one diamond with four diamonds in a balanced hand.
1 A linked factor to take into account is whether to use a cuebid over a weak-two as suggesting a solid suit and asking for a stopper, or as Michaels. If the former, then some people play jumps to four of a minor over a Multi or weak-two as Leaping Michaels – a forcing two-suiter with the bid minor and an(/the) unbid major.
2 Using 2H as the relay over two diamonds allows you to do this easily; but if you want to stick with 2NT as the relay, consider using 3 as any maximum, 3 as a minimum, and 3/ as natural.
3 A splinter is a posh way of saying a singleton or occasionally void
4 When you are in fourth chair and both opponents are bidding, you are ‘sandwiched’ between two active bidders – hence sandwich seat.