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.
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