The FIDE "laws of Chess" define check in terms of being attacked, and 'attack' in terms of pseudo-legal moves of the pieces. This does not allow for the pseudo-legal moves to change over time, and tacitly assigns the same pseudo-legal moves to a piece whether it is its turn to move or not. So it involves the fiction that the checking side can move, even if it is not his turn. To move out of turn you would either have to do two moves in the same turn, or let the opponent pass his turn between the two moves. This is how the null move enters the discussion.
Legality of moves in FIDE in general is completely covered by the rule "it is not legal to expose your King to capture, except for capturing the opponent King". Only castling is special, in that it is additionally declared illegal when the King starts or moves over an attacked square.
'Exposing to capture"is completely unambiguous. And it definitely does not indicate the interpretation you sketch above, where the Joker would "forbid a lot of King moves". The ambiguity is what moving out of check or through check means. Note that Fergus avoided any ambiguity by stating that the Royal Queen in Caïssa Britannia can not pass through a square that it could not legally move to". This also seems the logical way to treat passing through check in the context of castling. (Except perhaps for 3-step castling, where the second step in itself would be illegal because it is not even pseudo-legal, and the phrase would have to replaced by "where it cannot legally teleport to if teleporting was a pseudo-legal King move.) The spirit of the rule is all about the King being shot down during the attempt, like e.p. capture. Like I said, this is not a Joker-specific problem; many other pieces whose pseudo-legal moves depend on context (lame leapers, hoppers) require more precise definition of when castling is allowed.
Being in check at the beginning of your turn (and moving out of it) is yet another matter. Which affects both castling legality, and the stalemate definition.
To apply these definitions to a Joker, it becomes essential to define when exactly the pseudo-legal moves of the Joker change. It cannot be at the start of a turn, as at that point it is not yet decided which piece is going to move, so you cannot know what the new moves are. Declaring it has no moves at that point comes "out of nowhere"; no other piece loses all its moves (for determing whether the opponent would castle out of check) after it finishes is move. It seems much less arbitrary to let it keep its moves until another piece actually starts moving. That would affect whether you consider the side to move to be in check or not. And thereby whether castling is legal, or whether a terminal position is checkmate or stalemate. But nothing else.
The FIDE "laws of Chess" define check in terms of being attacked, and 'attack' in terms of pseudo-legal moves of the pieces. This does not allow for the pseudo-legal moves to change over time, and tacitly assigns the same pseudo-legal moves to a piece whether it is its turn to move or not. So it involves the fiction that the checking side can move, even if it is not his turn. To move out of turn you would either have to do two moves in the same turn, or let the opponent pass his turn between the two moves. This is how the null move enters the discussion.
Legality of moves in FIDE in general is completely covered by the rule "it is not legal to expose your King to capture, except for capturing the opponent King". Only castling is special, in that it is additionally declared illegal when the King starts or moves over an attacked square.
'Exposing to capture"is completely unambiguous. And it definitely does not indicate the interpretation you sketch above, where the Joker would "forbid a lot of King moves". The ambiguity is what moving out of check or through check means. Note that Fergus avoided any ambiguity by stating that the Royal Queen in Caïssa Britannia can not pass through a square that it could not legally move to". This also seems the logical way to treat passing through check in the context of castling. (Except perhaps for 3-step castling, where the second step in itself would be illegal because it is not even pseudo-legal, and the phrase would have to replaced by "where it cannot legally teleport to if teleporting was a pseudo-legal King move.) The spirit of the rule is all about the King being shot down during the attempt, like e.p. capture. Like I said, this is not a Joker-specific problem; many other pieces whose pseudo-legal moves depend on context (lame leapers, hoppers) require more precise definition of when castling is allowed.
Being in check at the beginning of your turn (and moving out of it) is yet another matter. Which affects both castling legality, and the stalemate definition.
To apply these definitions to a Joker, it becomes essential to define when exactly the pseudo-legal moves of the Joker change. It cannot be at the start of a turn, as at that point it is not yet decided which piece is going to move, so you cannot know what the new moves are. Declaring it has no moves at that point comes "out of nowhere"; no other piece loses all its moves (for determing whether the opponent would castle out of check) after it finishes is move. It seems much less arbitrary to let it keep its moves until another piece actually starts moving. That would affect whether you consider the side to move to be in check or not. And thereby whether castling is legal, or whether a terminal position is checkmate or stalemate. But nothing else.