The game uses the conventional rules of chess with the addition that when a piece is attacked by a piece of the same type but opposite colour (for example, a black queen attacking a white queen) it is paralysed and becomes unable to move, capture or give check.
Most of the time, two like pieces attack each other mutually, meaning they are both paralysed.
En passant pawn captures are an exception to this, since the attack is not mutual.
(The status of an en passant capture is open to debate, according to Pritchard.)
White then has the reply 2.bxc4 which is checkmate: Black has no safe squares for his king, he cannot capture the checking unit, he cannot interpose a piece between the checking unit and the king, and he cannot paralyse the checking unit (note that ...Rxe5 paralysing the c5 rook is not possible, because the g5 rook is paralysed by its counterpart on g2).