En passant (chess)
Both 'pieces' are in fact pawns. The rule was devised to prevent a player using the two-squares rule (that which allows a pawn to advance two ranks, from second to fourth, when first moving, should the player wish) to pass an enemy pawn.
The rule is: whenever a pawn advances two squares when moving for the first time, if by so doing it arrives on the same rank and adjacent to an opponent's pawn, that pawn may take it by occupying the square it would have been on had it moved only one square. Thus when a pawn moves from second to fourth rank, it is vulnerable to any opposing pawn on the fourth rank on an adjacent file, which may take it by moving diagonally to one's third rank (the opponent's own sixth rank).
An en passant capture must be effected immediately after the vulnerable pawn has moved: if the pawn is not taken at once, the option is lost.