  Photoelectron circular dichroism results from one-photon ionization of chiral
molecules by circularly polarized light and manifests itself in
forward-backward asymmetry of electron emission in the direction orthogonal to
the light polarization plane. To expose the physical mechanism responsible for
asymmetric electron ejection, we first establish a rigorous relation between
the responses of unaligned and partially or perfectly aligned molecules. Next,
we identify a propensity field, which is responsible for the chiral response in
the electric-dipole approximation, i.e. a chiral response without magnetic
interactions. We find that this propensity field, up to notations, is
equivalent to the Berry curvature in a two-band solid. The propensity field
directly encodes optical propensity rules, extending our conclusions regarding
the role of propensity rules in defining the sign of forward-backward asymmetry
from the specific case of chiral hydrogen to generic chiral systems. Optical
propensity rules underlie the chiral response in photoelectron circular
dichroism. The enantiosensitive flux of the propensity field through the sphere
in momentum space determines the forward-backward asymmetry in unaligned
molecules and suggests a geometrical origin of the chiral response. This flux
has opposite sign for opposite enantiomers and vanishes for achiral molecules.
