A splinter is a bid by responder (generally, although opener may splinter, also) that shows four card trump support, a singleton or void and opening hand values. It is generally made in response to a major suit opening, although it may be made in response to a minor suit opening.
Let's say your partner opens 1 Heart and you hold the following:
That's a pretty good hand, don't you think? It's one you'd open. If you didn't have a specialized bid, you'd probably bid 2 Clubs, then jump to 4 Hearts, depending on your partner's response. However, is there a way to tell Partner that you have a singleton diamond? What do you think? Can you think of a way? Maybe kick her twice under the table?
OK, that's illegal, so you don't want to do that. How do you tell partner that you have an unusual hand here and describe it in one bid?
Give up? The answer, as you might have guessed, is that you make a bid called a splinter. I love this bid. How do you splinter?
You make a double jump bid and bid your singleton (or void, as the case may be).A double jump is a bid that skips 2 levels of bidding. So if you open 1 Club and partner bids 2 Spades, that's a jump, but only skips one level (it skips 1 Spade). So a double jump would be a response of 3 Spades, because that skips two levels of bidding, 1 Spade and 2 Spades.
So in response to Partner's opening bid of 1 Heart, you bid 4 Diamonds (skipping 2 Diamonds and 3 Diamonds)! That tells partner three things in one bid:
1) You have a singleton or void in diamonds;
2) You have at least four cards in her opening bid, hearts; and
3) Your hand has opening values, at least 13 HCP.
All that information in one bid! The bid is obviously game forcing and it's exploring for slam.
The bid may also be made by opener. Assume you are opener with the following hand:
and you open 1 Club. Partner responds 1 Heart. You may jump to 4 Diamonds promsing 4 Hearts and a singleton (or void) Diamond. This can be a little risky since partner could be responding with only 6 HCP, so it shouldn't be made by opener unless opener has a strong hand with extra values, like the hand above that can play for game against 6 HCP. With the singleton Diamond and at least a 4-4 trump fit, this hand re-evaluates to 21 points, giving three for the singleton. When you have at least a 4-4 trump fit, shortness values increase to the following:
void 5 points
singleton 3 points
doubleton remains at 1 point
The problem with the bid is that if you're not used to it, your partner could misinterpret it and pass. Then you're playing the contract in your singleton (don’t laugh; it has happened to me). That's not much fun. So if you decide you want to play splinters be sure you and your Partner are on the same wave length.