In a two-center-shell-model description of asymmetric fission one works with a continuous asymmetry parameter ξ. After the scission point, however, the mass distribution can change only in steps of whole nucleons. A generator-coordinate consideration shows how ξ discretises in that it separates into intervals belonging to different mass distributions. At the boundaries of the intervals infinite walls build up which decouple the spaces dynamically, hindering any transfer. For practical purposes a parametrisation is given for the walls which have to be added to the usual collective Hamiltonian obtained from a cranking theory.