Suppose your RHO opens 1NT, and you make some call that shows a specific major and an unknown minor. Maybe Cappelletti. (Same thing would come up after Michaels and other similar competitive bids.)
LHO now makes some sort of call or pass. Does something, or doesn't. Whatever.
Partner now leaps to game in your major. All is good, except that one of the opponents bids again. I hate it when that happens (unless they are bidding like fools, which is always nice).
Anyway, you are sitting there, now, wondering what to do. Obviously, your hand is a player, but you really want partner's hand to fit your two-suiter. Wouldn't it be nice to ask partner if he likes the fact that you have diamonds rather than clubs? Damn problem is that you cannot bid those diamonds except at the five-level.
A possible auction:
Let's say, instead, that your partner bid 2S and you were the one who raised to 4S. Wouldn't you like to know if his suit is diamonds, which fits really well with your hand, or clubs, which sounds great for defense/penalty?
Seems like a problem. What to do, what to do?
Seems rather self-explanatory, eh?
Easy one there, too.
The idea to these should be (IMO) that this fit-jump doesn't show a trick source. What would we need that for? Rather, this fit-jump shows spades with "stuff" in the suit indicated. "I'm bidding 4S, partner. If you have diamonds, I fit really well with you. If you have clubs, think defense."