Weak jump overcalls is a convention for interfering with opponents' openings, leaving little room for communication. It has pretty much universally replaced strong jump overcalls.
A direct jump overcall of a non-preemptive suit opening is weak, showing at most 10 points, a 6+ card suit, and little defensive strength. In particular, the level of the bid should be two to five tricks beyond the number of playing tricks in the hand, depending on vulnerability (two with unfavorable, three with equal, three to five with favorable). SAYC recommends bidding at the same level one would open at if opponents hadn't opened.
Partner's responses after a weak jump overcall are
- any raise is a further preempt and must be passed.
- new suits are natural and not forcing, and possibly lead-directing.
- 2NT may be either a natural invite to 3NT, or else a forcing response, similar to weak two bids. This should be discussed.
- cue-bid of enemy suit is a forcing game or slam try.
- rebid suit with nothing, otherwise make any other bid.
- any game bid is a sign-off.
- any other jump in a new suit is natural/invitational.
There are a number of situations in which weak jump overcalls do not make sense. In the following situations, a jump overcall shows a 6+ card suit within one playing trick of the bid: