En Passant
It's the same as a mankhad in cricket. It is in the rules but only exists to enable you to show that you are a ttw at
The pawn then captures as if the opposing pawn had only moved one square.
An obscure historical reason for it…
No, I think the reason for it is that, when the rule that a pawn could move 2 squares on it's first move was added, it became too easy to create a passed pawn. As such, En Passant was made