  Velocities close to the speed of light are a robust observational property of
the jets observed in microquasars and AGNs, and are expected to be behind much
of the phenomenology of GRBs. Yet, the mechanism boosting relativistic jets to
such large Lorentz factors is still essentially unknown. Building on recent
general-relativistic, multidimensional simulations of progenitors of short
GRBs, we discuss a new effect in relativistic hydrodynamics which can act as an
efficient booster in jets. This effect is purely hydrodynamical and occurs when
large velocities tangential to a discontinuity are present in the flow,
yielding Lorentz factors $\Gamma \sim 10^2-10^3$ or larger in flows with
moderate initial Lorentz factors. Although without a Newtonian counterpart,
this effect can be explained easily through the most elementary hydrodynamical
flow: i.e., a relativistic Riemann problem.
