  This paper explores the effects of post-AGB winds driven solely by magnetic
pressure from the stellar surface. It is found that winds can reach high speeds
under this assumption, and lead to the formation of highly collimated
proto-planetary nebulae. Bipolar knotty jets with periodic features and
constant velocity are well reproduced by the models. Several wind models with
terminal velocities from a few tens of $\kms$ up to $10^3$ $\kms$ are
calculated, yielding outflows with linear momenta in the range $10^{36}-10^{40}
\gcms$, and kinetic energies in the range $10^{42}-10^{47} $ erg. These results
are in accord with recent observations of proto-planetary nebulae that have
pointed out serious energy and momentum deficits if radiation pressure is
considered as the only driver for these outflows. Our models strengthen the
notion that the large mass-loss rates of post-AGB stars, together with the
short transition times from the late AGB to the planetary nebula stage, could
be directly linked with the generation of strong magnetic fields during this
transition stage.
