What is Indian Castling exactly? Western castling doesn’t exist in historical Indian Chess (Chaturanga). Instead, in some versions, there is a cool move whereby each king can once in the game make a knight's move.
I read that it is can’t be used after check. But is that correct? Is it indeed a right which is lost for good after being checked, or is it something which cannot be exercised while the king is in check, but the right will return after the check no longer applies?
I guess I am hoping that it can be exercised even when in check, because it’s so cool to have a chance to escape otherwise certain loss - like James Bond.
How do the two kings interact directly? Must they avoid being a knight move apart. And if one has used its ability can the the other check it allowing bare K to checkmate e.g. K+N?
And how universal in Indian chess is this rule, please?
EDIT: The reason for this is that I think that Chaturanga can be interesting for chess problems, particularly retrograde analysis, but we need to agree a standard &quot;canonical&quot; version of the rules. The get-out-of-jail-free card is a unique mechanism like nothing else in chess, but I can't find a clear definition of how anyone uses it.
EDIT 2: Some relevant links:

https://en.wikipedia.org/wiki/Indian_chess
https://en.wikipedia.org/wiki/History_of_chess#Origin
https://www.chessvariants.com/historic.dir/hindustani.html
https://www.chessvariants.com/historic.dir/chaturanga.html

The last link given is the most relevant, see John Gollon's account. However, I submit that the text is not clear, and I'm looking for another primary source.
I was reading through &quot;American Chess Magazine Volume 1&quot; on Google Books, which is from 1897. There is an old article on the origins of chess therein, with the basic summary of how the game to be: it's reminiscent of today's Wikipedia as it was at the time. To my surprise, on page 264, there was direct mention of what you speak of! It occurs when speaking of chess's predecessor, Shatranj, which itself is a form of the much older, four-player game Chaturanga, aka &quot;Indian Chess.&quot;
Here is the relevant text.

The King was limited Shatranj, as in our game, to a move of one square in any direction, except that once during a game he was allowed the move of a Knight ; i.e, from his own square to King's Knight's second square, to King's Bishop, third square, to Queen's third square, or to Queen's Bishop's second square. This Knight move was omitted and Castling substituted.

At the end of page 265, it says:

In all variations of Chaturanga, the common origin is shown by the retention of one object, viz.: the capture of the opponent's King. When this is accomplished it is a checkmate, &quot;Shiek-Mate,&quot; the death of the King--the game is finished.

So there is a primary source, I suppose. It seems to me that the special knight move can only be used by an unmoved king. Also , reading into it, that an opponent can walk their king into a king's knight move range. Shatranj is a variation of Chaturanga, I think, and if the goal is to capture the king. So &quot;stalemate&quot; as we know it doesn't exist, and therefore so doesn't check. I admit that this interpretation of the check is my opinion, so it is still unclear, but what is clear is where on the board &quot;Indian Castling&quot; can take place.