Successful moves don’t always commute. Black has just moved his pawn from g7 to g5. White submits what he thinks is an en passant capture, fxg6. Black submits a knight move, Ng6. One order removes a Black pawn, the other a Black knight.
> Rule 2: a. Moves are tried in both orders, and only moves that are legal in both orders are merged. b. If both moves are legal in both orders but a different game state is reached in each order, neither move is merged.
> All of these scenarios illustrate rule 2a. Rule 2b is in fact irrelevant for chess, because successful moves always commute.
in the article.