  The physical processes that control the partition of released magnetic energy
between electrons and ions during reconnection is explored through
particle-in-cell simulations and analytical techniques. We demonstrate that the
development of a large-scale parallel electric field and its associated
potential controls the relative heating of electrons and ions. The potential
develops to restrain heated exhaust electrons and enhances their heating by
confining electrons in the region where magnetic energy is released.
Simultaneously the potential slows ions entering the exhaust below the
Alfv\'enic speed expected from the traditional counterstreaming picture of ion
heating. Unexpectedly, the magnitude of the potential and therefore the
relative partition of energy between electrons and ions is not a constant but
rather depends on the upstream parameters and specifically the upstream
electron normalized temperature (electron beta). These findings suggest that
the fraction of magnetic energy converted into the total thermal energy may be
independent of upstream parameters.
