  The possibility that mass ejected during Asymptotic Giant Branch (AGB)
stellar evolution phases falls back towards the star has been suggested in
applications ranging from the formation of accretion disks to the powering of
late-thermal pulses. In this paper, we seek to explicate the properties of
fallback flow trajectories from mass-loss events. We focus on a transient phase
of mass ejection with sub-escape speeds, followed by a phase of a typical AGB
wind. We solve the problem using both hydrodynamic simulations and a simplified
one-dimensional analytic model that matches the simulations. For a given set of
initial wind characteristics, we find a critical shell velocity that
distinguishes between "shell fallback" and "shell escape". We discuss the
relevance of our results for both single and binary AGB stars. In particular,
we discuss how our results help to frame further studies of fallback as a
mechanism for forming the substantial population of observed post-AGB stars
with dusty disks.
