  The cold dark matter (CDM) structure formation scenario faces challenges on
(sub)galactic scales, central among them being the `cusp-core' problem. A known
remedy, driving CDM out of galactic centres, invokes interactions with baryons,
through fluctuations in the gravitational potential arising from feedback or
orbiting clumps of gas or stars. Here we interpret core formation in a
hydrodynamic simulation in terms of a theoretical formulation, which may be
considered a generalisation of Chandrasekhar's theory of two body relaxation to
the case when the density fluctuations do not arise from white noise; it
presents a simple characterisation of the effects of complex hydrodynamics and
`subgrid physics'. The power spectrum of gaseous fluctuations is found to
follow a power law over a range of scales, appropriate for a fully turbulent
compressible medium. The potential fluctuations leading to core formation are
nearly normally distributed, which allows for the energy transfer leading to
core formation to be described as a standard diffusion process, initially
increasing the velocity dispersion of test particles as in Chandrasekhar's
theory. We calculate the energy transfer from the fluctuating gas to the halo
and find it consistent with theoretical expectations. We also examine how the
initial kinetic energy input to halo particles is redistributed to form a core.
The temporal mass decrease inside the forming core may be fit by an exponential
form; a simple prescription based on our model associates the characteristic
timescale with an energy relaxation time. We compare the resulting theoretical
density distribution with that in the simulation.
