  We show that when the relevant CP violating interactions in leptogenesis are
fast, the different matter density asymmetries are determined at each instant
by a balance condition between the amount of asymmetry being created and
destroyed. This fact allows to understand in a simple way many features of
leptogenesis in the strong washout regime. In particular, we find some
non-trivial effects of flavour changing interactions that conserve lepton
number, which are specially relevant in models for leptogenesis that rely
heavily on flavour effects.
