Hybrid-code simulations of slow shocks indicate that there exists a critical number, Mc, such that slow shocks with an intermediate Mach number MI≥Mc have a rotational wave train and slow shocks with MI<Mc do not have such a wave train. We propose an ion heating mechanism of slow shocks, which is associated with the chaotic motion of particles in the downstream wave field. For a coherent electromagnetic wave propagating along the downstream magnetic field, corresponding to switch-off shocks, the particle motions are not chaotic. For an oblique wave, the interaction between the particles and the wave field may lead to chaotic particle motions. Such particles may be greatly thermalized within one wave-length after they are incident into the downstream wave field. The results can be used to explain the existence of the critical intermediate Mach number observed in they hybrid simulations.