Playing Matchpoints, South deals and open 1S.
The auction without Bergen Raises:
West passes in the first round of the auction. With insufficient number of HCP West cannot make an overcall bid in the first round. However, when the auction is about to end in 2S, West must consider a balancing action (which is not free of risk when vulnerable). The singleton in spades is the main reason to bid in the balancing seat. West also holds working HCP for the offense: QJ10 in the main suit, an Ace and a King. If West decides to bid, he/she ought to consider which is the correct balancing bid to maximize the chance of the partnership to find a fit: should West choose the strain, bidding hearts, or let partner choose the strain bidding a takeout double? Often with 5-4-3-1 (a 3-suited distribution) a balancing double is the better call.
East's response to double in this auction is an advanced convention. Instead of guessing which of the minor suit to bid in response to the balancing double, the use of 'Unusual 2NT' (AKA 'Scrambling 2NT') is not natural but artificial showing 2 places to play. Then the partners will bid suits up the line to find the fit. In this deal, using both conventions: the balancing double and the Unusual 2NT allows E/W to locate the D fit. Other contracts are doomed to go down lacking a fit.
Who should be the one to compete to 3S? North or South? USUALLY the one who holds the extra trump (the 9th trump). HCP do not matter when deciding whether to compete, but distribution and the SIZE of the fit. North competes to 3S.
The auction playing Bergen Raises:
When the N/S partnership plays Bergern Raises the auction is simple. North responds 3C showing 4 spades with (6)7-9 HCP. South signs off in 3S.
Playing 3S contract
West leads the HQ. Declarer wins with the HA. How should declarer continue?
Identifying NO potential side winners, declarer must NOT draw trumps, but plan instead to ruff 2 clubs in the dummy. If trumps divide 2-2 (40%), drawing two rounds won't hurt declarer who can still reach 9 winners by ruffing twice in the dummy. But if trumps divide 3-1 (50%), declarer must plan to ruff before drawing trumps. Note that declarer can also succeed on 'Dummy Reversal', ruffing 3 times in his/her own hand. In any case, declarer should not touch trumps. Playing even 1 round of trumps can be a mistake if the defense is alert and continues playing trump whenever in turn to play.
From the defense's perspective, whenever declarer avoids drawing trump, the defense should consider playing trumps. This guideline is intuitive and is a part of the main defensive strategy of countering declarer's play. If East is alert, he/she will lead trumps whenever in turn to play. Since declarer has to lose 3 tricks (2 in clubs and 1 trick in the red suit (to allow immediate transportation for the second ruff), it appears that defense can prevail by playing 3 rounds of trumps anytime in turn.
But West cannot lead a trump after the first round, holding only 1 card. Declarer will use this fact to make the contract by using a special avoidance play in clubs. The order of play:
Trick 1: declarer wins with the HA
Trick 2: declarer plays a second heart, losing to East (this play is important to prepare the transportation for the second ruff in the dummy).
Trick 3: East plays spade (best defense). Declarer must win in the DUMMY for the next play.
Trick 4: declarer starts clubs from the dummy, covering East's card to force West -who has no trump cards remaining- to gain the lead to the next trick.
West does not have a trump to play next. Declarer gains the tempo to be able to ruff 2 clubs in the dummy. Declarer will then continue clubs and can ruff twice in the dummy:
Trick 5: West plays a diamond. Dummy wins the DA.
Trick 6: declarer plays a second club. East wins.
Trick 7: East plays a second trump.
Trick 8-9-10 cross ruff!
Note that East play of the CK in trick 4 will not help, as the CA will drop in the third round establishing declarer's CQ.