Here's a recent deal from a regional Swiss:
A simple auction lands you in 6NT, and West leads the ♣7: ♣9, ♣10, ♣K.
What's your plan?
You have 10 top tricks, and you will always have an 11th from diamonds, even if you misguess the ♦Q. If that occurs, your 12th trick could come from a squeeze, a 3-3 club break (or ♣J10-doubleton), or a miracle in hearts (♥J109-tripleton).
How do you plan to play the diamonds? Make sure you have a contingency plan if you lose to the ♦Q.
Assuming you lose a diamond trick, you should play for a squeeze. The fourth round of diamonds will be a winner, so you can't have a squeeze threat in that suit. A simple squeeze in any two suits will operate against either player, provided that player solely guards both of those suits. To squeeze a player in spades and clubs, you will reach:
If someone holds the ♣J along with five or more spades, he will have unguarded one of those suits. Ditto for the club-heart squeeze:
Note that this ending also caters to a double squeeze played as a simple squeeze. Only one player can guard hearts, but you don't care which if the club and spade guards are split. If one of the black cards in dummy is a winner, cash it. Otherwise, try to run the hearts. Note that if one defender can guard spades and clubs and the other has four-plus hearts, the squeeze will not operate.
What about a double squeeze with spades as the commonly-guarded suit? If East holds the club guard, which, if clubs aren't 3-3, is overwhelmingly likely given the lead (who leads the ♣7 from ♣J7xx(x) against 6NT?), the squeeze won't work.
In the hypothetical matrix, East guards clubs, West guards hearts, and both guard spades. The flaw is that both singly-guarded threats are under their respective guards, so when you attempt to execute the double squeeze (effectively two simple squeezes), one of your hands will be squeezed before the relevant defender. Here is a possible position:
When the ♦A is cashed, East is squeezed out of a spade, but West can simply discard from the same suit that South does.
This is all well and good, but what does it suggest about the correct way to play 6NT?
The main takeaway is that if you lose a diamond finesse, your squeeze chances aren't great. You should try your best to avoid losing a trick to the ♦Q. Which player should you play for that card?
Consider West's opening lead, which is presumably from two, three, or four low cards. There is an element of restricted choice in that selection: if West had all low cards in clubs and diamonds, in theory he would be equally-likely to select a diamond lead as a club. That West actually chose to lead a club increases the likelihood that West has the ♦Q.
What if West bizarrely led from ♣J7xx? Does his choice of lead suggest that East holds the ♦Q, either because he has more vacant spaces in his hand or on restricted-choice grounds? Hardly—who leads a club from jack-fourth against a blind auction to 6NT instead of a diamond from two, three, or four low?
All evidence suggests playing West for the ♦Q. Before you commit to that, how are your chances if you win a finesse but diamonds don't break?
If you successfully finesse East for the ♦Q (against the odds, but let's examine it anyway), you're not home yet. If East holds guards in both minors, a simple squeeze won't work against him, because both threats are underneath him. You could try to squeeze West in the majors, but that requires ducking a minor-suit trick to rectify the count. You can't duck in a major, because that erases your threat in that suit. Let's say you test diamonds, find them 2=4, and give East the fourth round of the suit (you discard a spade from hand). Win the (say) spade return in hand and cash the last club (finding East with the guard). That leaves:
That resembles the layout on page 2. If East began with 3=2=4=4 distribution and hence can guard spades, you're out of luck. Playing for different squeezes present similar difficulties. I haven't gone through every possible variation. If you can find a sure-tricks line if East guards both minors, more power to you—I didn't see one at the table, and I don't see one now.
What if you successfully finesse West for the ♦Q? You're cold. Ice cold. If either minor plays for four tricks, that's 12. If West guards both minors, you have a simple squeeze against him. (But you have to be careful to finesse in diamonds without cashing the ♦K first, because when you duck a spade to rectify the count, West might be able to win and return the ♦Q, cutting your communication in the minors and killing the squeeze.)
If East guards clubs, you'll have a double squeeze (actually, a simple played as a double). Unlike the previously-discussed double squeezes, this one is a lock. Hearts is the doubly-guarded suit (more accurately, the suit you don't care who guards), and you can duck a spade, cash the minor winners, and reach:
(If the opponents return a heart, the same position develops with the ♥Q gone.) If neither minor-suit card in dummy is a winner, then someone will have gotten squeezed out of the heart guard.
We're getting close to the end. Let's go back to trick two (actually, three, after you cash the ♣A):
Lead a diamond to the jack. If it wins, duck a spade, then test the minors (clubs first, in case you're going to be simple-squeezing West in the minors). Take the top spades, and the squeeze almost plays itself.
At the table, getting diamonds right and then playing for the squeeze was necessary, because the layout was:
Among other things, I found this deal interesting because it seemed to combine lots of declarer-play themes, from drawing restricted-choice inferences to projecting different potential squeezes at trick two. It was also nice to put some of the principles in Bridge Squeezes Complete to use at the table. It's been a while since a single deal I've played has contained so many possible (and impossible) squeezes.
Plus... it's free!