  We present an alternative model for the formation of fast low-ionization
emission regions (FLIERs) in planetary nebulae that is able to account for many
of their attendant characteristics and circumvent the problems on the
collimation/formation mechanisms found in previous studies. In this model, the
section of the stellar wind flowing along the symmetry axis carries less
mechanical momentum than at higher latitudes, and temporarily develops a
concave or inverted shock geometry. The shocked ambient material is thus
refracted towards the symmetry axis, instead of away from it, and accumulates
in the concave section. The reverse is true for the outflowing stellar wind,
which in the reverse shock is refracted away from the axis. It surrounds the
stagnation region of the bow-shock and confines the trapped ambient gas. The
latter has time to cool and is then compressed into a dense "stagnation knot"
or "stagnation jet". In the presence of a variable stellar wind these features
may eventually overrun the expanding nebular shell and appear as detached
FLIERs. We present representative two and three-dimensional hydrodynamic
simulations of the formation and early evolution of stagnation knots and jets
and compare their dynamical properties with those of FLIERs in planetary
nebulae.
  Subject Headings: hydrodynamics - ISM: jets and outflows - ISM: kinematics
and dynamics - planetary nebulae: FLIERs and stagnation knots
