This is a defense convention against preemptive opennings, Developed by Harry Fishbein. Double against any preemptive openning is for penalty. Any suit overcall shows at least 4 cards lowest suit. It promises 16+ HCP and a triple suited or at least double suited hand. For instance over 3 Spades openning, if he has 5 cards Hearts and 4 cards Clubs he should bid 4 Clubs. His partner bids his longest suit in possible lowest level with a weak hand, bids game with 9+ points, makes Cue-Bid with 12+ points.
This convention is not applied after two passes.