  We report the mechanism of the hysteresis in the transition between Regular
and Mach reflections. A new discovery is that, the hysteresis loop is in fact
the projection of a higher dimensional path, i.e. the valley lines in the
surface of dissipation, of which minimal values correspond to stable reflection
configurations. Since the saddle-nodes bifurcate the valleies of the surface,
they are actually the transition points of the two reflections. Furthermore,
the predicted reflection configurations agree well with the experimental and
numerical results, which is a validation of this theory.
