En passant is a legal move.
en passant

not exactly i think it does like a knight move but know that i think about it i'm wrong aren't i?
[http://en.wikipedia.org/wiki/En_passant. mod.]

Say you're playing the white pieces. You have a pawn on e5. Your opponent has a pawn on f7, and tries to move her pawn to f5 in one move. You have the choice to take that pawn by removing it from the board and placing your e5 pawn on f6.
why aren't en passants legal in chess games? is it just too O.P or what?