Once, one of your pawns on the a- or h-file may step one square diagonally forward onto an empty square. Not a capture.
You draft Cellar Hatch in Buff mode. Passive: it works on its own the whole time you hold it, with nothing to activate.
As a movement card, it changes how one of your pieces is allowed to move.
“The edge files hide a trapdoor under the barrels.”