This method is quite popular amongst players of 2/1 GF: after a major-suit opening, a jump to 3 of a lower suit is invitational and shows a good suit (6+ cards). Personally I think this is a truly dreadful idea.
The problems aren't too difficult to spot. It's a very space-consuming response. This means that it will make subsequent bidding difficult, and you can't afford to make the bid on too wide a variety of hands.
In my experience the invitational jump-shift is almost guaranteed to put opener in a difficult position. Let's try these very ordinary hands, after the bidding starts 1S : 3D -
Here perhaps partner can stop the hearts and you can run lots of tricks in NT. Or perhaps he can't. How are you going to find out, with no bid available below 3NT?
Here if partner has heart support you might belong in 4H, but alternatively the hand could be a complete misfit not making anything higher than 3D. Make the hand a little stronger and you know you should be in game, but will you find 4H if that's the best spot? How will partner know you have five hearts?
A lovely hand in support of diamonds, but how do you distinguish this hand from all the other different shapes that might want to raise?
Of course, if you play a 2/1 as absolutely forcing to game, you do need somewhere to put these invitational hands. But not all invitational hands with a good suit are suitable for an IJS. It is dangerous to make an IJS on hands with tolerance for parnter's major, or with four cards in the other major, because of the possibility of missing a major-suit game. So playing IJS does not really solve the problem of what to do on these hands.
Much better, at least when one of these invitational hands comes up, is to be playing a system where a 2/1 is not absolutely forcing to game, with responder's rebid of his suit showing the invitational type. Starting at the lower level gives you so much more flexibility: not only does opener have an extra chance to describe his hand, but responder can make the bid on hands which are not such pure single-suiters, because he is not committed to showing the IJS type. For example, over 1S, hands with four hearts and a six-card minor are no longer a problem, because after a 2/1 response, responder can afford to raise a heart bid to game, and will only rebid his minor to show an IJS hand if opener does not have hearts.
Naturally this would make life more difficult when you do actually have a game-forcing single-suiter. But personally I think it is relatively easy to find a way to bid these hands. Even if constrained to play natural methods, I would much prefer to be playing 2/1 "GF except rebid" than absolute game-force. And with a bit of artificiality there is plenty of room in most cases to distinguish invitational from game-forcing hands without having to invent suits or NT bids. Certainly in a natural GF system, responder's 2-level rebids (when available) tend to be underused, and can be redefined to include the game-forcing single-suited type. Really I think that using a cheap response like 2C solely for natural game-forcing hands is a serious waste of space.
An alternative for people who want to keep their 2/1s as game-forcing is putting the invitational single-suiters into the 1NT response. This has several drawbacks. You don't get to show either the suit or the strength immediately, and if you later bid your long suit it might be difficult to distinguish this from a weaker hand. Also if 1NT is not forcing, you may well be missing a better contract if opener passes. It works better over a 1H opening than over 1S - particularly if playing Kaplan Inversion so that the "forcing NT" hand bids 1S rather than 1NT - because responder has a 2S rebid available as artificial to distinguish weak single-suiters from invitational ones. Even then, I still prefer making a two-over-one response, showing the suit immediately, if the system can be arranged to allow for it.