Greg Strong wrote on Wed, Mar 11, 2020 12:49 AM UTC:
"Fast castling" isn't just the King and Rook changing places - the Rook goes to the King square but the King can go anywhere in between so long as it is not attacked. But squares in between can be attacked and need not even be empty.
This comes from Kevin Pacey's Waffle Chess. Here is his description:
A king that has never moved, and is not in check, can 'leap' once a game, along the first rank, to any unattacked empty square between it and an unmoved rook, followed by said rook 'leaping' to the king's initial square so as to complete castling in one single move. It does not matter if any squares in between are occupied or under attack.
"Fast castling" isn't just the King and Rook changing places - the Rook goes to the King square but the King can go anywhere in between so long as it is not attacked. But squares in between can be attacked and need not even be empty.
This comes from Kevin Pacey's Waffle Chess. Here is his description: