This follows on from the explanation of the artificial 2C response to major-suit openings. If playing natural 2/1 responses (apart from 2C), 1S : 2D and 1S : 2H would both promise five-card suits. This works fairly well, but there is a lot to gain from switching the 2D and 2H responses round, so that 1S : 2D shows hearts.
Clearly this will improve the bidding of hands with hearts at the expense of hands with diamonds. But because of the importance of major suits, this seems to be worthwhile.
The most important gain is on hands not worth forcing to game.
Suppose that you respond to 1S with 1NT on this hand, and partner rebids 2D. This gives you a difficult rebid problem. You could try 2S or 2NT, but neither of these show your lovely heart suit. Alternatively you could rebid 2H, but this doesn't express the strength of the hand very well. This is the sort of hand that the 2D transfer was designed for. You respond 2D on this hand, and if opener completes the transfer by bidding 2H, you continue with 2S. The 2S rebid is non-forcing and mildly invitational. Not only does this sequence get the heart suit into the picture, it also perfectly describes the strength.
The 2D bid should promise a decent hand. The minimum strength is about the same as a traditional Acol 2H response:
This hand is just about worth a 2D response: this time we will pass opener's 2H rebid. However, when holding only five hearts and shortness in spades, it is better to respond 1NT unless the hand has genuinely invitational values (that is, it should be good enough for a 2NT rebid).
Of course, game-forcing hands with 5+ hearts must respond 2D as well. After opener's minimum rebids, they have to bid a minor suit in order to create a forcing auction, like in Acol.
Opener is not obliged to complete the transfer, but, apart from raising hearts, the only alternative with a minimum hand is to bid 2S. This shows 6+ spades, and will tend to be short in hearts (but if the spade suit is sufficiently good opener might have heart tolerance). Opener's minimum rebids are not forcing: responder would pass with the minimum 8-point hand above. With a stronger hand, opener bids as in Acol, except that I would play 2NT as showing a good hand with 6+ spades (forcing to game).
As was said above, the problem with playing 2D as a transfer is that you have to work out what to do when responder actually has diamonds. Balanced hands with diamonds are not a worry, since they can be put into the artificial 2C response. But unbalanced hands with diamonds have to respond 2H to 1S. The loss of a step can make life difficult - especially from opener's point of view, when he holds hearts. Some further artificiality is likely to be needed to try to deal with these problems.
An alternative approach is to put all the hands with diamonds into the 2C response. This frees up 1S : 2H to be used as a spade raise. (And similarly, 1H : 2D could be used as a heart raise.) I rather like this idea, but it means that the 2C response becomes incredibly complicated, though there does seem to be just enough room to make it work. I wrote out some notes for a complete system of artificial 2/1 responses here.