The requirements of the Laws include:
So where are we? Clearly we have to explain agreements with our partners: clearly we do not have to explain if we have no agreement. But how do we look at the grey areas?
We have two extremes. At one end we have people who are unwilling to explain what they know. Their reasons may be good, or may be bad. Some of them claim to do so because their agreements are 'general' rather than special. Some of them claim to do so because they may give UI to partner with their answers. Some of them claim to do so because they have not discussed the actual sequence. Some of them claim too many questions is harassment. Some of them try to win at the game by leaving their opponents in the dark.
None of these arguments has much going for it. A 'special' agreement is one for the partnership rather than general bridge knowledge. An example of a 'special' bridge agreement is that 2 over 1NT is Stayman with this partner. It may be general knowledge that it is Stayman with most partners, but I know of three partnerships who do not play it: they play 2 as Keri, Gladiator and Transfer to hearts, respectively. So it is a 'special' agreement that you play Stayman with this partner.
Suppose you have not discussed it. When you sit down with a strange partner and say "Acol, RKCB, double for takeout over pre-empts, transfers and Sputnik" there are a lot of agreements that you are making without saying so explicitly. You would not say Transfers and not mention anything else if you were not playing Stayman. Once you have said this and partner agreed you have an implicit agreement that you are playing Stayman.
As for the UI argument, you must play to the Laws of the game. Law 75A makes your agreements fully and freely available to opponents: if this gives UI to partner that is tough. Nothing in the laws makes it an infraction to give UI to partner.
The harassment argument has something going for it, and the TD will protect a player if necessary. But harassment comes when the question has been answered fully and the questioning continues. Players that do not answer fully must expect questioning to continue.
How about the other extreme? How far should you explain? The answer is that you should explain your agreements fully and freely. This includes implicit agreements based on experience of the partnership, similar sources for the partnership, comparable sequences, and so on.
But there it ends. The proposal that you should invent some sort of agreement is illegal. It has been suggested that it might be helpful to the opponents. That may or may not be true, but it is a matter for the law-makers, not individual TDs or players who are not prepared to follow the Laws. An explanation that provides something that is not an explicit or implicit agreement is illegal under Law 75 and should be dealt with accordingly.
Specifically we should consider the answer "No agreement". If there is none of course this is the right answer, but great care should be taken to make sure there is no relevant partnership experience or implicit understanding that is relevant.
Despite the fact that some people have good motives - not everyone, by any means - players who will not answer questions about partnership understandings freely and fully have broken the Law, and should be dealt with: this applies whether they have provided insufficient answers, or too much by inventing agreements that do not apply to this partnership.