  Recent symmetry considerations (Phys. Rev. Lett. {\bf 84} 2358 (2000)) have
shown that dc currents may be generated in the stochastic layer of a system
describing the motion of a particle in a one-dimensional potential in the
presence of an ac time-periodic drive. In this paper we explain the dynamical
origin of this current. We show that the dc current is induced by the presence
and desymmetrization of ballistic channels inside the stochastic layer. The
existence of these channels is due to resonance islands with non-zero winding
numbers. The characterization of the flights dynamics inside ballistic channels
is described by distribution functions. We obtain these distribution functions
numerically and find very good agreement with simulation data.
