  The physics of collisionless relativistic shocks with a moderate
magnetization is presented. Micro-physics is relevant to explain the most
energetic radiative phenomena of Nature, namely that of the termination shock
of Gamma Ray Bursts. A transition towards Fermi process occurs for decreasing
magnetization around a critical value which turns out to be the condition for
the scattering to break the mean field inhibition. Scattering is produced by
magnetic micro-turbulence driven by the current carried by returning particles,
which had not been considered till now, but turns out to be more intense than
Weibel's one around the transition. The current is also responsible for a
buffer effect on the motion of the incoming flow, on which the threshold for
the onset of turbulence depends.
