GAME-TRY (Maximal) DOUBLES
Also known as a maximal double, this double is used by opener to make a game try. This allows opener the ability to compete to the three level without inviting game. Suppose you hold:
A2 KQ10984 43 K32.
At favorable vulnerability, you open 1H and partner raises to 2H. You’d love to buy it there, but RHO bids 3D. Since your side has 9 trumps, you want to compete to the three level, so you bid 3H. But, what if partner thinks you have an invitational hand and he raises you to 4H? This is why the Game-Try (maximal) double exists. In the auction described, 3H means what you want it to mean (“Partner—I am just competing for the partscore”). If instead, you did have a game-invitational hand, such as:
AQ3 KJ987 3 AJ102,
you would double. This call says nothing about diamonds. It just says: “Partner, I have a 3 1⁄2 heart bid. Please go to four with a maximum, but retreat to 3H with a minimum.”
Anytime your side has raised an opening from 1-of-major to 2-of-a-major and Opener’s RHO interferes in the suit directly under the major, then:
Double=Artificial game-invitational hand (saying nothing about the suit being doubled)3-of-your-major=Just competing for the partscore.
This is a dangerous convention, since disaster looms if either player forgets the agreement. Be careful to note that this applies only if the overcall is exactly one under. If the auction goes: 1H-P-2H-P-3C, for example, double would be for penalties. There will be room (in this case, 3D) for an opener who wishes to make a game try (though such a try would say nothing about diamonds, since it is the only game try available).
There are other auctions where some experts use this convention (such as after they’ve overcalled and raised).
SOURCE: http://larryco.com/BridgeArticles/ArticleDetails.aspx?articleID=553
(Lead-Directing, Maximal and Other Doubles)