All pawns positioned behind the Line of Sovereignty may move forward two spaces on their initial move. Another player's piece or pawn may capture such a moved pawn "en passant" only on any immediately following move (as made by each following player), if desired.
Capturing "En passant" (Fr. In Passing) can be accomplished only when a pawn is moved two spaces on its initial move. If an opponent pawn (in this case, the Red pawn pictured below) is positioned such that the initial pawn (the Blue pawn) could have been captured if it had been moved only a single space, then the Red pawn may capture the Blue pawn on Red's next move (and only on Red's next move) as though the (Blue) pawn had been moved only one space--the opponent (Red) removes the (Blue) pawn from its placement on the board, and places the (Red) capturing pawn where the (Blue) initial pawn would have been placed had it been moved forward only one space.