In a round-robin match in the trials, an enemy preempt gives you a difficult decision.
N-S vul, South deals. As South, you hold:
If you are going to bid a suit, 4♣ is definitely better than 3♠. Even though partner's double implied support for the majors more than clubs, bidding a 3-card suit can't be right when you have a good 5-card suit to bid. If you bid 3♠, you might wind up in a 3-3 fit.
3NT is a possibility. Your diamond stopper is such that you can hold up if necessary long enough to exhaust East's diamonds and keep West from running the suit. The problem is that you are fairly weak. A diamond lead could knock out the only entry to the club suit, and if the club suit doesn't run it is hard to imagine coming close to taking 9 tricks.
Passing the double is also a possibility. Favorable vulnerability preempts can be quite frisky, and the easiest route to a plus score might be to contract for 5 tricks. Still, you can't be that confident of defeating 3♦ doubled, and if you pass and they make, that is very costly. Also, you certainly won't collect the value of your vulnerable game if you have a vulnerable game.
The percentage call looks to be 3NT. It isn't that you expect 3NT to make. In fact, 3NT probably won't make. The point is that when 3NT does make you get the big payoff of that vulnerable game bonus. If you could be sure of getting a plus score by bidding 4♣ or passing the double you would prefer to do that. But, since there is no sure plus score available, you might as well collect the money when 3NT is right.
You bid 3NT, ending the auction.
West leads the ♠10, standard leads, and partner puts down a pleasant dummy.
What do you play from dummy at trick 1?
It looks like you are in pretty good shape. Barring bad splits, you figure to take at least 2 spade tricks, 1 heart trick, 2 diamond tricks, and 4 club tricks.
The ♠10 appears to be top of a doubleton. It must be right to cover with the queen. This will retain maximum fluidity in the spade suit for the future. You will even have a spade entry to your hand should that be important. Nothing East can do will damage you much.
You cover with the ♠Q. East wins the king, and returns the ♣2. What club do you play from your hand?
If the clubs are 4-2 you will definitely have 9 tricks, so you are only concerned about a 5-1 club split. Naturally you can't play a high club, as that would block the suit. If you play the ♣8 and it holds, you are home. You can lead a club to the ace, and then a small spade from dummy, making full use of your spade spots to get a second entry to establish and run the clubs. Similarly, if you play the ♣6 and it forces the ♣9, you can win the ace and set up the clubs. So, it is a guess whether to play West for a singleton 7 or a singleton 9. There isn't much in it, but East would perhaps have been more inclined to shift to a club from K9xxx than from K7xxx, so the 8 is probably the better play.
You choose to play the ♣6. West covers with the ♣7, and you win the ace. What do you do now?
It looks natural to continue clubs. This will always make unless the clubs are 5-1.
You lead the ♣3 from dummy. East plays the ♣4. What club do you play from your hand?
While it seems quite possible that East started with K9xxx of clubs, it isn't worth staking the contract on that. Even if the clubs are 5-1, you will have plenty of ways to score your ninth trick. Losing to West's ♣9 could be a disaster.
You play the ♣Q. West shows out, pitching a diamond. Naturally you continue with the ♣J, pitching a heart from dummy. West pitches another diamond. East wins the king, and shifts to a diamond. Where do you win this diamond shift, and what will your plan be?
You don't need entries to dummy, since the ♥A will always be there. Retaining the diamond entry to your hand may be important.
Your plan will be to win the ♦K, and lead a low spade off dummy. Hopefully, all East will be able to do is take his jack and return a spade. You will then cash the good spades (nice 7-spot so you can overtake), play a diamond to your ace, cash the ♣10, and throw East in with a club. He will be forced to lead a heart and give dummy the last 2 tricks.
What can go wrong with this plan, and what will you do about it?
The greatest danger is that East has a second diamond. He will lead it when he is in with the ♠J. This will prevent you from executing the end-play, since you won't be able to cash the spades and then get back to your hand to throw East in. You will now be reduced taking 2 heart finesses. The odds on this will be pretty good since West's shape will be 2-4-6-1. You will win the diamond shift, ride the ♥9, win the spade return in your hand, cash the ♣10, and take another heart finesse. This loses only when East has ♥KJ doubleton.
The other danger is that East started with 5 spades. This is unlikely since that would make West's distribution 1-4-7-1, and from that he probably would have led a heart in preference to a spade. However, if that is the case the squeeze-endplay won't work. East will return a spade. If you cash your spades and come to your ♦A as before, East will discard a heart, and you won't know whether or not he has blanked his king. Once again, your best bet is simply to win the spade return in your hand and take two heart finesses, losing only when East started with ♥KJ doubleton.
As it happens, none of the dangers materialize. East wins the ♠J, West follows, and East returns a spade. As planned, you overtake and cash the good spade, come to your hand with the ♦A, cash the ♣10, and throw East in with a club. The full hand:
The chosen line of play looks pretty solid. Do you see any way it could have been improved (other than guessing the clubs at trick 2)?
An improvement would be to lead a small spade off dummy at trick 3 instead of a club. This will make a difference when West's shape is 2-4-6-1. East wins the spade, and shifts to a diamond. You win in dummy, cash the spades, and play on clubs. Now the endplay has to work. The difference is that you were able to cash the spades before your ♦A got knocked out.
This approach also works better when West's distribution is 1-4-7-1. East wins the spade, and returns a spade. You win in dummy, and lead a club which East must duck. Now you pass the ♥9, which in the worst-case scenario loses to the jack. East can do no better than play another spade. You win and simply play ♥A and ♥Q, setting up the long heart and scoring 3 heart tricks, 2 diamond tricks, 2 spade tricks, and 2 club tricks. The defense gets 2 heart tricks, and 2 spade tricks, but they don't get the ♣K.
Finding this line of play at the table is difficult, since setting up the clubs immediately looks so natural. However, if you are into the hand and looking at all the possibilities the spade play at trick 3 becomes quite clear.
What do you think of West's opening lead?
It looks reasonable. There can't be much future in the entryless diamond suit. South probably doesn't have 4 spades since he bid 3NT, so there is a good chance of West finding his partner with a 5-card spade suit. If East had held 5 spades, or better spade spots, the spade lead could have been a big winner. As it was the lead was helpful to declarer's line of play, although the contract almost certainly would have been made on a diamond lead since East would have been repeatedly endplayed.
Do you agree with the 3♦ opening bid?
3♦ is the book opening bid. But at this vulnerability and with this particular diamond suit, it looks insufficient. It is unlikely that E-W belong in 3NT. Opening 4♦ puts more pressure on N-S, which could cause them to make a double-figure IMP error. It is true that 4♦ goes down 1 more trick than 3♦, but the opponents still have to judge to defend.
The actual deal is a good illustration. The 4♦ opening catches East with about the worst hand type -- diamond shortness and secondary stuff in all the other suits. And the 4♦ opening is still the winner. If N-S do everything right they can double and collect 500 (3 side aces, 2 top trump tricks, and a heart ruff), but there is plenty of room for N-S to go wrong both in the bidding and the defense after a 4♦ opening bid. Even if NS gets it all right, -500 is better than the -600 for defending 3NT.
Hamman's rule says that if there are several possible options and 3NT is one of them, 3NT should be chosen. This rule applies even more strongly vs. an enemy preempt. The actual deal is a perfect example. A 4♦ opening would have robbed N-S of the opportunity to employ Hamman's rule.
Plus... it's free!