  The energetics of the long duration GRB phenomenon is compared with models of
a rotating Black Hole (BH) in a strong magnetic field generated by an accreting
torus. A rough estimate of the energy extracted from a rotating BH with the
Blandford-Znajek mechanism is obtained with a very simple assumption: an
inelastic collision between the rotating BH and the torus. The GRB energy
emission is attributed to an high magnetic field that breaks down the vacuum
around the BH and gives origin to a e+- fireball. Its subsequent evolution is
hypothesized, in analogy with the in-flight decay of an elementary particle, to
evolve in two distinct phases. The first one occurs close to the engine and is
responsible of energizing and collimating the shells. The second one consists
of a radiation dominated expansion, which correspondingly accelerates the
relativistic photon--particle fluid and ends at the transparency time. This
mechanism simply predicts that the observed Lorentz factor is determined by the
product of the Lorentz factor of the shell close to the engine and the Lorentz
factor derived by the expansion. An anisotropy in the fireball propagation is
thus naturally produced, whose degree depends on the bulk Lorentz factor at the
end of the collimation phase.
