  We propose an asymmetrical jet ejection mechanism in order to model the
mirror symmetry observed in the lobe distribution of some protoplanetary
nebulae (pPNe), such as the pPN CRL 618. 3D hydrodynamical simulations of a
precessing jet launched from an orbiting source were carried out including an
alternation in the ejections of the two outflow lobes, depending on which side
of the precessing accretion disk is hit by the accretion column from a Roche
lobe-filling binary companion. Both synthetic optical emission maps and
position-velocity (PV) diagrams were obtained from the numerical results with
the purpose of carrying out a direct comparison with observations. Depending on
the observer's point of view, multipolar morphologies are obtained which
exhibit a mirror symmetry at large distances from the central source. The
obtained lobe sizes and their spatial distribution are in good agreement with
the observed morphology of the pPN CRL 618. We also obtain that the kinematic
ages of the fingers are similar to those obtained in the observations.
