There are some considerations that must be applied to the subsequent bidding after responder bids 2 Diamonds.
As mentioned in the last article, it is important to remember that opener is much less limited in strength and distribution in an XYZ auction (compared to the 2-way checkback auctions). When the auction begins with 1 Club - 1 Heart - 1 Spade, opener may have any of these hands (and anything between them):
What this means is that the context of the auction after a 2 Diamond bid by responder is not in which opener is trying to move out of responder's way, to allow him room to describe. Both players may have features requiring description (and both players may preempt the other's plan). The good news is that the auction is really at a very low level, and since it is forced to game, both players can (and should) try to preserve his partner's options by conserving bidding space. Jumps should be used only when absolutely necessary -- and we will discuss why and when sometimes they are necessary.
Using that auction as a template (1 Club - 1 Heart - 1 Spade - 2 Diamonds), let us look at the possible actions by opener.
2 Hearts: 3-card support. This bid must be chosen whenever opener has 3-heart support, even with very bulky hands (e.g. 4-3-0-6), because is conserves so much space, and the message of secondary support is always useful for responder, even if his intention is not to play there. It is important to observe that, depending on your agreements about this auction (check here for a full discussion), your hand will very often be balanced (4432, or even 4333). Responder cannot draw any inference (yet) about diamond shortness or stoppers.
2 Spades: The natural meaning of this bid (rebidding a side suit) would be to show 5 spades, hence, a black 65 (some people allow an 1 Club opening with 55). That is fine but all too rare. We need to use this cheap bid more often, to conserve our bidding spade. My suggestion is to use it as a petty, little odious bid (PLOB - readers of The Bridge World will recognize the reference), i.e., as a default bid for all hands that do not fit neatly in any of the other bids. In other words, all that responder will take from this bid is a lack of 3-card heart support.
2 NT: no 3-card heart support, and diamond stoppers. Not necessarily balanced, but opener will only pick this bid with unbalanced hands if this is really the best description of the hand, i.e., very nice diamond stoppers, bad club suit. For example: AQxx x KQ10 Jxxxx is a 2NT bid. If the hand were changed only a little bit into AQxx x Qxx KJxxx I would probably prefer to bid 2 Spades and wait for partner to clarify his intentions. This preference for showing the balanced nature of a hand with a notrump bid simplifies many auctions.
3 Clubs: 6 cards in clubs, denying 3 hearts. The 2 Spade PLOB bid allows us to give much firmer definition to higher bids. Showing 6 cards in clubs without any ambiguity is important if responder is looking for a slam.
3 Diamonds: 4 diamonds if your system allows it (4=1=4=4). If you would always open 1 Diamond with a 3-suiter and both minor suits (would you? Discuss it with your partner!), this is an idle bid, and probably the best use for it is to show a 4=1=3=5 hand with a semi-stopper in diamonds. (If you have no semblance of a stopper in diamonds you can bid 2 Spades and commit to notrump only if your partner bids that strain).
3 Hearts: You would not do badly if you never bid 3 Hearts or anything higher. Remember, responder may still have a bulky hand to describe. But if you need to define this bid as something my suggestion would be a minimum (not a maximum, which would often inspire a slam search) with exactly 4=3=1=5. This would allow some neat negative inferences when opener does not use this route, and responder would only miss the space gobbled up by the jump if he has an enormous hand (he can always bid spades or clubs to set these suits as trump, or 4 Diamonds if he wants to set hearts as trump).
Responder's next actions are hard to map out, since we have too many variations. Natural bidding should be employed. When should anyone jump at this juncture or later? With slam invitational hands. Note that this does not include slam forcing hands. In other words, there are hands that are too strong for later jumps. What a jump conveys, in a forcing auction such as these, is "I am not sure about slam, what do you think?" Proper slam bidding includes taking some chances and elevating the level of the bidding, sometimes to the level above game. But if you never jump with these hands you will never find the 16-16 slams, since the minimum of both partners is about one Ace lighter than that.
Your partnership should discuss what jumps in "impossible suits" should mean. Shortness, Void, Exclusion are the most common meanings. One example would be 1 Club - 1 Heart - 1 Spade - 2 Diamonds - 2 Hearts - 4 Diamonds. (Observe that any big two-suiter with responder does not begin the exploration with 2 Diamonds. You have a 2NT tool, over 1 Spade, to describe two-suiters).