This Real Deal comes from the South Florida IMP Game. The West player held:

1098
AJ9543
109
AJ

With nobody vulnerable, his RHO dealt and opened with a weak 2-bid in diamonds. Would you overcall?

There is an old saying, "you can't preempt a preempt." That means that a jump-overcall of 3 would not be preemptive here; it would actually be intermediate (about a 16-count and a decent 6-card suit). While you aren't worth 3, I think it is okay to scrape up a 2 overcall. Generally, I like to have opening-bid strength to make a 2-level overcall, but I'm going to upgrade for my six-card suit and good spot cards. After committing the 2 overbid (er, I mean, overcall), LHO raises to 3, passed back to you. Surely, you've done enough, so now you get to lead against 3.

I'm not a fan of laying down aces (though when RHO has preempted, and the strength rates to be on your left, it's not a bad idea). We'll see how you would have done later if you're an ace-layer-downer. Let's choose the sequence instead and go with the 10.

You see:

Vul: None Dir: South	AQJ2 K102 Q2 5432
1098 AJ9543 109 AJ	--

Dummy wins the A, partner playing an encouraging 7. Next comes the Q, a diamond to the ace and the K, partner following up the line. You throw a heart. Declarer produces another spade, you follow and partner wins the K, the first trick for the defense. Partner shifts to the 7 (standard carding), declarer plays the 10 and I ask you for your plan.

Maybe without a warning, you would have lazily won your J (as a national champion did at the table). There is no longer any way to defeat the contract. All the signs you needed are there to make the winning play of the A then the J.

Declarer is already marked with AKJxxx for six tricks. He won the A at trick one, which gets him up to seven tricks. Dummy is sitting with a good spade and the K for nine. How are you going to set him if you win your J? All you will get is your AJ, A and partner's K. You need to hope declarer has three club losers. Partner tried to help you by playing all of his diamonds up the line (low, next lowest, then the last one). With nothing better to do with those three little diamonds, he was trying to tell you he had good cards in the lowest ranking suit. Furthermore, your only chance was that he held the KQ (not a surprise given declarer's weak two-bid). Here was the Real Deal:

Vul: None Dir: South	AQJ2 K102 Q2 5432
1098 AJ9543 109 AJ		K763 86 753 KQ87
	54 Q7 AKJ864 1096

In 3, declarer won the spade lead with dummy's ace, drew trump and played another spade to dummy's jack and East's king. When East returned a low (4th-best) club, declarer played the 10 and the moment of truth had arrived. If West reflexively (and cheaply) took his J, it was the end of the road for the defense. Clubs were blocked and declarer could get to dummy's K to throw a club loser on the good spade. If West correctly wins the A and continues the suit, the defense can cash 5 top tricks for down one.

How would your ace-bashing on opening lead have done? Probably not so well. After the A lead, declarer can unblock his Q and then has an easy path to 9 tricks (6 diamonds, one spade and two hearts). After the A lead, the contract can be defeated, but West has to find a spade shift (at once, or by ruffing his partner's good third round of clubs) in time. It's more challenging with the actual spade lead--that way we can check West's reflexes to see if he can resist the knee-jerk reaction of cheaply winning the J.