En Passant
The principle of this unique pawn rule is that by moving two squares immediately, your pawn never had the chance to capture it. The rule was to reduce the likelihood of locking pawn walls where nothing could pass through unless it was a knight or forcing sacrificing more valuable pieces in order to break through the structures easily, as well as reducing the abuse of the unique two squares forward at a time pawn move rule in general, acting as a counter balancing rule. Pawns are the most unique pieces in the chess game, having the most rules and various movements and it’s often why initially chess players learn about pawns before any of the other pieces in the game.