It's a legal move called "en passant". En passant (from French: "in [the pawn's] passing") is a maneuver in chess when a player moves a pawn two squares forward from its starting position, and an opposing pawn could have captured it if it had only moved one square forward. The En passant rule was introduced in 1490.http://www.chess.com/chessopedia/view/en-passant