Hands are from La Jolla Unit Game, October 14, 2018.
With very distributional hands, both sides may end up competing to the five and even the six level, not sure to whom the hand belongs. Sometimes there are double-game swings with both sides able to score a game. A major principle is such circumstances is to describe your hand as fully as you can so your partner can make a more informed decision—in terms of doubling, passing, or bidding one more.
West: ♠— ♥K1053 ♦7643 ♣AQ854
East: ♠AJ3 ♥AQ9872 ♦10 ♣KJ2
South passed and West passed and North made a NON-standard third seat opening of 1♠ when she had only four spades to the King and a perfectly good ♦KQJx to open if she wished to open light. East overcalled 2♥ (promising an opening hand for a 2-level overcall). South jumped to 4♠—distribution, little defense. West can bid 5♥, but that does not help partner know what to do if someone bids 5♠.
West's best bid is 5♣. Partner will know that he is NOT trotting out a new suit at the five level, particularly as a passed hand. So, that shows an excellent heart raise, and a decent side suit in clubs. Partner, holding the East hand can envision ♥Kxxx and ♣AQxxx, and easily find a 6♥ call whether the opponents bid 5♠ or not. East expects her two losing spades to go away on partner's good clubs—plus partner is pretty much guaranteed to have a singleton or void in spades on this auction. If West simply bids 5♥, East will double 5♠ by South. Collecting 500 (an 8% result in practice) is very poor versus your 680 (19%), 710 (62%) or 1430 (96%). Furthermore, if you DO defend 5♠ doubled and partner has bid clubs, you can lead your singleton diamond. Then you grab the first spade and put partner in with a club for a diamond ruff. Then get to partner again with the ♥K for another diamond ruff. Plus 800 will beat all the people who stop in game, for a 92% board in practice.
Bidding the slam gets you 12½ out of 13 matchpoints. Most pairs made seven (+710) because the opponents (naturally) led a spade rather than a diamond. You ruff one spade in Dummy and discard two spades and one diamond on the good clubs.
Matthew Kidd says: I highly recommend the book Partnership Bidding at Bridge: The Contest Auction by Andrew Robson and Oliver Segal, which discusses many competitive bidding situations similar to this one. It is also the rare bridge book that has been released to the public. I can send you a PDF version.
South (Dummy): ♠AQ7632 ♥Q9 ♦KJ72 ♣K
West: ♠4 ♥1084 ♦AQ8 ♣J108643
North opened 1♥. South responded 1♠. North rebid 2♥ and South jumped to 4♥. Partner led the ♦3 (fourth best).
This is matchpoints, not IMPS, so you have to decide whether you can set the contract, or you are just trying to avoid giving away too many overtricks. You are entitled to two diamond tricks because Declarer has exactly two cards in the suit (rarely three, if partner has led from ♦10xx). If partner has the ♣A, you are entitled to a club trick as well. If Declarer has the ♠Kxx, you must cash out immediately. If, however, partner has the ♠K10xx of spades, the spade suit is NOT running immediately, and you may want to cut down on club ruffs in Dummy. If Declarer has the ♣A, the ♣K will be another Dummy entry and allow Declarer to set up the spade suit even if he has only a doubleton.
So, the most likely scenario is that Declarer is going to do very well on this hand. You expect Declarer to have ♥AKxxxx and either the ♠K or the ♣A—and possibly both. If he has both, you are not getting anything other than your two diamond tricks, so it is definitely time to cash and NOT to worry about setting up the ♦KJ in Dummy for later discards. Partner's second diamond, if it is low, should suggest a club back, and that is the maximum you will achieve. [If partner's second diamond is high or looks like a middle card, she probably does not have anything other than the ♣Q.]
Holding Declarer to ten tricks would net you 11 out of 13 matchpoints for an 85% board. Letting them make 6 (with a club or heart switch) gets only 4½ matchpoints (35%).