Fit Showing Jumps 
You hold this hand and partner opens 1, what do you bid? This may depend upon your style/system; with 3, 4, 4 (splinter) or 3 (whatever that means in your system) all being possible candidates.
But now suppose that RHO sticks oar in with a double or a 1 bid, does this change things?
The opponents probably have a fit and it is by no means certain that your side will win the auction. If the opponents end up declaring then you want partner to lead s when he gets in. And if partner also has s then he can compete to a high level. The solution is the ‘Fit Jump in Competition’. Regardless of what 3 would have meant had there been no intervention (natural weak, natural strong, mini splinter, Bergen raise or whatever) the jump bid now is best used as showing a 4-card fit with partner and an excellent suit (the one bid).
So the jump shift after partner opens a major and RHO overcalls or doubles is defined as a limit raise or better with 4-card support and a good outside suit.
With the hand given, a 3 bid over the overcall/double is far more descriptive than some sort of splinter. These jump shifts in competition are single jumps, double jumps are splinters.
Fit jumps opposite an overcall
Jump shifts in competition may also be used if partner overcalls. Suppose LHO opened 1, partner overcalled 1 and RHO bids 1. You can again bid 3 - a jump shift showing good s and 4-card support.
Fit jumps by a passed hand
Fit jumps may also be used by a passed hand when there has been no intervention. You hold the same hand and pass as dealer (or 2nd seat) and partner opens 1 in 3rd or 4th seat. You could use Drury but a fit jump of 3 is more likely to get you to the best spot.
If you have a tendency to open lightly in 3rd maybe some form of Drury is better. This is all down to what your partnership decides..
Fit Showing Jumps 
Fit showing jumps (FSJ) can be played in all seats and by a passed hand as well as over interference. The major purpose of Fit Showing Jumps is to discover double fits that are important when deciding whether to bid on in competition and in discovering slam possibilities. In deciding whether to use a fit showing jump, you and partner need to abide by some rules. Let’s note the following suggested guidelines:
- Concentrated strength in both suits (partner's suit and yours);
- No outside strength in other suits;
- Your suit should have two of top three honors;
- At least three or more cards in partner's suit -- if only three cards then must be headed by an honor.
Opener K 7 A Q 9 7 3 2 Q 5 2 A
Pard 5 4 K J 6 A K J 7 6 3 4 2
The auction might proceed:
1 1 3* 4
4NT Pass 5 Pass
6 All Pass
*Fit Showing Jump
Opener can practically count 12 tricks. Diamonds should be headed by two of top three honors and five plus cards in the suit you jump to. This also shows three plus cards in opener's suit and if only three cards, one must be an honor.
The main feature of this convention is that you have the ability of knowing where your tricks are coming from when you have a double fit. Indeed FSJ are very useful with an overcall: W: 1 N: 1 E: Pass: S3*. What else could 3* be but heart support, a limit raise or better and a good five-card club suit? Now North holds:
A 6 Q J 10 9 8 J 10 A Q J 10
Game looks very enticing indeed!