Source: Justin Lall Blog
Nobody is vulnerable and you pick up 
K9742 
74 
6 
KJT87. LHO passes, partner opens 1 and RHO makes a takeout X. You’re not sure how to describe your hand, but luckily you have you have a 4 gadget at your disposal to show 5+ clubs, 4+ spades and a game going hand. In fact, this would be the textbook hand for that auction. This “fit jump” enables partner to accurately judge what to do if the opponents compete to the 5 level and can get you to a low HCP slam opposite a prime double fitter. It’s the perfect bid right?
WRONG!
A bid being the perfect description of your hand does not necessarily make it the right bid. Though in constructive auctions I advocate describing your hand as well as possible, competitive bidding is a completely different matter. Tactical approaches that will make life hard for the opponents should often be adopted, even at the cost of some accuracy.
The biggest problem with the 4 bid is that it gives LHO entry to the 4 level to introduce a save-suggesting bid. For example, with a hand like 63 KQ853 Q542 94 LHO can bid 4 to invite his partner to save in 5. This way they get to communicate and judge intelligently whether they should be bidding at the 5 level or not. Had you just bid an immediate 4 your left hand opponent would be forced to pass with that hand, and your RHO would also pass without significant extra values.
The other problem with 4 is that you aren’t just telling your partner what your hand looks like, you’re telling the opponents. That information gives them an even greater ability to judge whether to bid on, double you, or pass. If RHO has a balanced 18 count with most of his values in the red suits, he would have to double back in if you jump to 4. However, by jumping to 4 you alert him that his red suit tricks are not cashing so he will no longer feel compelled to double you. Likewise, LHO will not X 4 with a random 10 or 11 after a 4bid unless he has values in the black suits. He will even know that his red suit values are likely to be working on offense and may save or suggest a save that he wouldn’t have done after the more straight forward jump.
What about the potential downsides of disguising your hand with an immediate jump to 4? If the opponents do go on to the 5 level then you will be poorly placed. You will guess right some of the time, but not as much as you would have had you started with 4. This risk is not as great as it seems though. With LHO being a passed hand he is unlikely to have a long enough suit or a good enough hand to bid at the 5 level. If he does bid, it will most likely be a card showing X which will generally be passed out at such a high level. You also might miss a slam, but that would be a rare occurrence after RHO has shown opening values. Partner would need to have a great fit, and in that case the opponents may have one as well and might have a cheap save at the 7 level.
It is worthwhile to note that the problems of making a fit jump in this auction are so severe because the suits involved are spades and clubs. If you made the hand 6 K9742 KJT87 74 after partner opens 1 and RHO Xs, a 4bid would have a lot more appeal as you now give the opponents very little additional useful room. Since you need to prepare for the likely 4 bid it now becomes a good tactical maneuver to describe your hand. However, in the original example a fit jump is clearly a tactical blunder.