Lesson
White to move. Main line: 1. hxg6#
hxg6 en passant moves the white pawn off h5 and removes Black’s pawn from g5. Vacating h5 opens the rook on h1 all the way to the king on h8, so the capture delivers a discovered rook check.
The bishop on c4 controls g8 and the king on f6 controls g7. Black cannot capture the rook, block the h-file, or move to either neighboring square. The FEN’s g6 en-passant field records that ...g7–g5 was the immediately preceding move.
En passant can alter two files in one move because the captured pawn disappears from a different square than the capturing pawn occupies. When it is available, redraw every rook and bishop line after both pawns have changed.