Instead of describing the thrump double, the decision was made to quote the author. The following description is by Mr. Marty Bergen, (website), who invented and devised the thrump double and its application. In case the visitor or bridge student would like to contact Mr. Marty Bergen, he/she can do so by visiting his website.
Thrump doubles, as described here, were invented by yours truly. While they certainly are different, experience has shown that they are essential at the three level.
What is a thrump double and why do we need them? The discussion of the bidding on this hand will clue you in.
Q7 Q2 1087 AKQJ75
Once your partner has opened the bidding, you are thinking about 3 No Trump. In fact, I will bet that is the case regardless of which suit your partner opened. You will show your Club and points and hope partner bids No trump sooner or later.
Nice plan. Now suppose that your RHO jumps to 3, 3, or 3 before you are able to make your first bid. Are you going to give up on the 3 No Trump contract you were heading for? If you make the normal 4 bid, do you expect partner to provide the five tricks you will need to bring home 5?
What is my suggestion? Make a negative double. However, instead of defining it as looking for the unbid Major(s), think of it as looking for THRee notrUMP. On most hands where partner has a stopper in the opponent’s suit, you would like him to bid 3 No Trump.
When is this needed? When the enemy’s natural jump overcall reaches 3, we cannot necessarily show our suit without going past 3 No Trump. Here are the 10 auctions where the opponent has made a natural, preemptive jump overcall above 3 and below 3 No Trump.
Must you have a solid suit? Absolutely not. A Thrump Double would be totally appropriate after 1 – (3) with:
864 65 AQ754 AQ2
93 K7 KQ10865 KQ8
63 A2 852 AK10854
Now you are the responder after 1 – 3.
K3 84 AKQJ5 J753
Double, hoping that partner bids 3 No Trump.
KQ863 KQ2 K7 985
Forget your Spades. You have Hearts stopped, so bid 3 No Trump.
AK A64 AK83 J852
Once in a blue moon you will have a great hand with a stopper in the opponent’s suit. In that case, you can start with a Negative Double and then explore for slam.