A weak-two bid is a most effective pre-emptive and descriptive bid. Since two clubs represents a very strong bid in any suit, two diamonds, two hearts and two spades can be used for weak hands. Typically they are six card suits with two of the top three honours or three of the top five honours with five to nine points.
Having the ability to draw trump reasonably quickly is rather important. Also being able to withstand partner’s being very short in the weak two suit is also important. Unless partner is strong, this is the suit to compete and it should survive being doubled opposite partner’s void in the suit.
Therefore, any bid by partner in another suit is forcing and constructive.
Other constraints are no four-card major, no void and no distribution that makes it equivalent to a one-opening or stronger. With a void or four-card major, the hand becomes more valuable than partner will expect from a weak two bid. The idea is to preempt the opponents not one’s own side.
A weak two bid is not only valuable as a pre-empt, it also gives partner an effective description of one’s hand that will help him to bid his own hand. For example, with a singleton or void in partner’s suit, one should recognize the existence of a misfit and not bid too high.
South with a reasonable six-card spade suit and seven points, opens a weak two spades. East takes action and North jumps to four to jam the auction. West competes to five hearts which is really not a brave bid. It is a must considering the vulnerability and the fact that East and West have most of the points.
South does not have any more bids other than pass. He has described his hand and has no idea whether North’s four-spade bid was to make or just to further the pre-empt. North is the captain and decides to sacrifice with a singleton diamond and partner’s presumed heart shortness.
The contract: Five spades doubled by South
The opening lead: The king of diamonds
The play: West wins the opening lead, and seeing a stiff diamond in dummy switches to the king of hearts which South ruffs.
South ruffs two diamonds and takes the spade finesse with dummy’s last trump. It wins. South loses two clubs and a diamond for down 1. As the cards lie, East can make a small slam, but West cannot on a spade lead.
The result: Five spades doubled down one for -100
The result without weak two’s: Six hearts by East making for +1430.
Note: East and West with each having two spades decided to take a positive score instead of risking six hearts. South’s weak two and North’s subsequent bidding were very effective in preventing East and West getting to their makeable slam.