I’m sure that this has been covered before, but I recently had occasion to read the rules closely, and they seem somewhat contradictory. It’s one of those things that I thought I knew until I read the rule book.
(References below refer to the printed 5th edition rule book.)
Rule 8A2 describes the procedure for castling. Move the king two squares, then move the rook. However, it refers the reader to 10I2, in which the rook is touched first.
Meanwhile, rule 9A describes the rules for when a move is determined. If a piece is transferred to a vacant square, and then released, the move is determined with no possibility of change.
Suppose white has the intention to castle. He picks up the rook on h1, moves it to f1, and lets go. According to 9A, he has, at that point, executed a transfer to a legal square, and let go of the piece. It should be determined with no possibility of change. Furthermore, he was not following the procedure described in 8A2 for castling.
However, if that is the case, then how can we make sense of the existence of 10I2? The sensible way to touch the rook first while castling would be to move it to f1, then pick up the king and move it to g1, but that runs afoul of rules 9A and 8A2.
Surely the rulebook is not suggesting that one might touch the rook on h1, leave it positioned on h1, then pick up the king and move it to g1, then pick up the rook and move it to f1. It would be silly to specifically call for that move in the rulebook. Even sillier would be to pick up the rook on h1, put it down in an illegal position (like off the board, perhaps) then move the king to g1, then grab the rook and move it to f1. Silly or not, that is the only way I can make sense of the rules as written.
What was the actual intent?