Yesterday I was playing with some friends and at one point I got the following hand:
We're NV vs V, RHO opens in 1st seat with 2NT (20-21). What do you bid and why?
I thought opponents might have 4♠ or a slam in ♦, so why not mess up their auction. If they bid to 4♠, the contract will be wrongsided and partner can immediately give me a ruff. Perhaps I can even reach his hand for another ruff for -1. So I bid 4♦ which turned out to be a success. If opponents don't double (unlikely) I'll be happy to go -9, otherwise I can safely return to 4H. It's always exciting to bid natural with a void, especially if you expect partner to have lots of support and he should have the discipline not to raise...
The entire auction went:
2NT - 4♦! - Dbl - pass
pass - 4♥ - Dbl - pass
pass - pass
Now the psych has been exposed, but opponents can't make things right. South did a great job not to bid 4♠ at any time, and take his money on 4♥x down as much as possible.
The full hand:
As you can see, when ♠ is played by South a ♦ lead will defeat the contract if I can manage to play low when declarer plays ♠ (I should be able to find that, but we'll never know for sure). 4♠ by North is laydown. 4♥x-2 cost us 300, which is way better than 4♠=. Some however got to 6♠ or 6♦, I've even seen a 6NT!