The player must do en passant on the very next move after the opposing player makes the necessary two-square pawn advance. If not done, the ability to capture that advanced passed pawn by an en passant capture is lost forever.
The mechanics of en passant capture. When an opposing player advances a pawn two squares vertically on the same file from that pawn's home square to the same rank in which the player's pawn is located on an adjacent square in the file either to the immediate right or left of file in which the opposing player's pawn is moved, the player may capture the opposing pawn via en passant. Thus, the player's pawn must be on the opposing player's fourth rank [vertically upwards - 5th rank for White, and vertically downwards - 4th rank for Black].
The bypass avoids a normal capture if the pawn had been advanced only one square. Precisely for this reason, the en passant capture was developed as an option to counter the power of making a two-square advance and bypassing normal capture.
How is it accomplished? If an opposing player makes such a two-square advance during the game, the player optionally may capture the pawn that has been advanced two squares by advancing his or her adjacent pawn diagonally to the square immediately behind and in the same file as the pawn which was advanced two squares, and removing the advanced pawn from the chessboard. The capture thus involves advancing the pawn to capture as if it had done so normally on the square upon which the advanced pawn would have been captured if it had been advanced only one square. For White, this means moving his or her pawn to the appropriate square in the sixth rank, and for Black moving the pawn to the appropriate square in the third rank. An en passant capture example is diagrammed below.
En passant DIAGRAMS (4 board positions):
En passant Capture on f file
Note: Java required to use the following board; click here for a non-Java (HTML) review
