First I'll pose this as a single dummy problem, since it's quite interesting as well. A bit further down I'll show it as double dummy, but I suggest you try to solve it single dummy first.
Nobody vulnerable, you open 2♠ and the auction continues:
2♠ - pass - pass - 4♥
pass - 5♥ - pass - 6♥
You obviously start with ♠K and you get the following in dummy:
Declarer ducks, your partner and declarer both following. Now it's up to you: how will you try to defeat this contract? (you can count on declarer's playing skills)
Hint: your first thought should be WHY declarer ducked the first trick.
I won't give you the solution just yet, but I'll point you into the right direction. Declarer is probably rectifying the count for a squeeze. Having done this, he's now on schedule to perform a squeeze if there is one. This leaves out a ♠ or a ♥ switch.
Another option may also be that both you and declarer are quite smart, so he's trying to fool you into a minor suit switch. This is not the case here however.
Ok, so you know you have to switch to a minor. Which one?
If you think you know it, have a look at the entire hand:
Are you still sure about what you have to play at trick 2?
I asked on the Bridgebase Forums what people would play, and most said they think a ♦ is necessary. Lets consider all options:
- we continue with ♠Q:
declarer will ruff, draw trumps (unblocking), ruff a ♠ if necessary and come to the following ending:
On the play of the last 2 ♥s West will have to abandon a minor suit while dummy discards a ♦ and a ♣. Now ♦A is played and West has to leave with his other minor, while on the play of ♠A East is squeezed in both minors.
- we continue with a ♥ or a ♦:
The same ending can be reached as above.
- we continue with ♣Q:
This will get us to a double squeeze: West has to guard ♠, East has to guard ♣, and both opponents need to guard ♦s:
- we continue with a small ♣:
This is the only way to defeat the slam! There is no way declarer can make West discard something he doesn't want to. East just needs to keep his ♦s:
The funny part of this hand is that if you swap ♦9 and ♦8, the squeeze will function again, because you'll be able to finesse ♦ on the way back to your hand. In that case, you better lead a minor instead of ♠K, otherwise you gave away the slam...