The logic of an auction can call for a "denial cuebid" even though that has never been discussed. A case in point is a slight tweaking of a prior post here.
Suppose that Responder bids 4NT as 1430 RKCB for spades, in an auction where the partnership has definitely ruled out the possibility of a grand. Suppose that Opener's response is 5C, contextually showing three key cards. Suppose that contextually this must include the Ace or King of trumps but that Opener has already denied two top trumps (perhaps a prior 2NT cuebid, for example).
Now, as mentioned in the prior post, 5D cannot ask about the Queen of trumps, as that is already known -- Opener has already denied the spade Queen. If Opener has already denied a diamond control, then, as mentioned earlier, 5D must be asking for a tertiary control, the Queen, to protect his King-empty.
How does Opener answer?
If you think of Last Train as a "denial" cuebid of sorts, this becomes fairly easy. A Last Train call, in a sense, is a "denial" of sufficient extras to embark on the next course. It seeks something from partner.
Well, a similar concept occurs here. There are three ways to bid after 5D. Opener could deny the diamond Queen by bidding 5S, a sign-off. Or, he might bid 6S, accepting (or perhaps some esoteric other call that offers a choice of slams or suggests 6NT). Third, and key, Opener could bid 5H.
Now, as Responder, in this auction, is asking the questions, it makes little sense for 5H to be taken literally as a "Last Train" call, because that implies a vantage point that is errant. Opener is responding with Responder as captain; Last Train is used by the captain.
Rather, it seems to me that 5H should be a denial cue, showing the diamond Queen (a positive response) but "denying" ability to accept a hypothetical Last Train call. Accepting the slam try (or making an esoteric choice-of-slams call) implies ability to accept the hypothetical Last Train call.
Look that this another way. After 5D in the example, Opener would bid 5S (sign off) without the diamond Queen. If Opener has the diamond Queen, he will make a positive answer. Opener's task is then to imagine that he could make an insufficient bid of 5D to answer yes, allowing Responder to bid 5H as Last Train. If Opener would accept that Last Train call, Opener bids the slam. If Opener would decline, he bids 5H, below 5S and indicating that he would decline a Last Train call. Responder may have enough anyway and continue.
What this does is to allow Responder to ask Opener is he has the diamond Queen and that "something else" that would make the slam worth bidding, all with the one 5D call. If all Responder needs is the diamond Queen, he will not care about that "something else."
If you recall the problem (this month), Opener held Qxxxxx in spades and the diamond Queen. After the 5D asking bid, Opener could bid 5H if he had held the diamond Queen but only Qxxxx in spades. With the diamond Queen and a sixth spade, however, he has that "something else" to justify bidding slam.
Once accepting, however, Opener might suggest 6NT by bidding 6C (if he had AJ in clubs), 6D (if he held QJ in diamonds), or 6H (if he held AJ in hearts). There is no cost to this, and this probably should be considered. Except, I suppose, that partner's head might explode.