Twice, one of your pawns on the b- or g-file may step one square straight backward onto an empty square. The narrower district runs the errand twice.
You draft Squire's Errand 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 b- and g-pawns trot back to fetch the good lance.”