  The Inverse First Ionization Potential (FIP) Effect, the depletion in coronal
abundance of elements like Fe, Mg, and Si that are ionized in the solar
chromosphere relative to those that are neutral, has been identified in several
solar flares. We give a more detailed discussion of the mechanism of
fractionation by the ponderomotive force associated with magnetohydrodynamic
waves, paying special attention to the conditions in which Inverse FIP
fractionation arises in order to better understand its relation to the usual
FIP Effect, i.e. the enhancement of coronal abundance of Fe, Mg, Si, etc. The
FIP Effect is generated by parallel propagating Alfv\'en waves, with either
photospheric, or more likely coronal, origins. The Inverse FIP Effect arises as
upward propagating fast mode waves with an origin in the photosphere or below,
refract back downwards in the chromosphere where the Alfv\'en speed is
increasing with altitude. We give a more physically motivated picture of the
FIP fractionation, based on the wave refraction around inhomogeneities in the
solar atmosphere, and inspired by previous discussions of analogous phenomena
in the optical trapping of particles by laser beams. We apply these insights to
modeling the fractionation and find good agreement with the observations of
Katsuda et al. (2020; arXiv:2001.10643) and Dennis et al. (2015;
arXiv:1503.01602).
