You must be in the right position for this to work:
3. You
♠ 5 4 LHO Partner RHO You
♥ 8 7 5 2♣ Pass 2♦ Pass
♦ J 9 6
♣ A Q J 9 4
You want a club lead, but are in the wrong position to ask for it. If you double, you’d be indicating a diamond suit. And if you bid 3♣, you’re asking for trouble!
4. You
♠ 5 4 3 LHO Partner RHO You
♥ void 2 NT Pass 3♥* Pass
♦ K J 9 6
♣ 9 7 6 4 3 2 *Jacoby transfer for spades
If they wind up in spades, you’d love a heart lead. And the 3♥ bid is certainly artificial, so why not double? Two reasons: partner will believe you have length and may sacrifice - that is, bid hearts at a high level to keep them out - or if they wind up in notrump, he’ll still think you want a heart lead.
But here is an exception to the length requirement:
The double of a 4-level or higher artificial bid requires strength, but not length. A strong 3-card holding (K Q 10) is enough.
5. You
♠ J 7 5 4 LHO Partner RHO You
♥ 9 1♥ Pass 3♥ Pass
♦ K Q J 4 NT Pass 5♦ Dbl
♣ 9 7 6 4 3
The opponents are headed for a heart slam. You would love partner to lead a diamond, and now you have a lucky opportunity to tell him.
When you do NOT double a Blackwood response, you indicate that you have no interest in that suit.
How do you use this negative inference?
6. You
♠ 5 4 3 LHO Partner RHO You
♥ 8 7 5 1♦ Pass 1♥ Pass
♦ J 9 6 3♥ Pass 4 NT Pass
♣ 9 7 6 4 5♣ Pass 6♥ All Pass
You have two unbid suits to lead: spades and clubs. If partner wanted a club lead, he had an opportunity to double the 5♣ Blackwood response. So using negative inferences, try a spade lead. It’s just a guess, but an ‘intelligent’ guess!
