  Numerical simulations combined with simple analytical arguments are used to
reach a number of important conclusions on the nature of the Blandford-Znajek
mechanism. We show that, just like in the Penrose mechanism and in the MHD
models of Punsly and Coroniti, the key role in this mechanism is played by the
black hole ergosphere. The poloidal currents are driven by the gravitationally
induced electric field which cannot be screened within the ergosphere by any
static distribution of the electric charge of locally created pair plasma.
Contrary to what is expected in the Membrane paradigm, the energy and angular
momentum are extracted not only along the magnetic field lines penetrating the
event horizon but along all field lines penetrating the ergosphere. In dipolar
magnetic configurations symmetric relative to the equatorial plane the
force-free approximation breaks down within the ergosphere where a strong
current sheet develops along the equatorial plane. This current sheet supplies
energy and angular momentum at infinity to the surrounding force-free
magnetosphere. The Blandford-Znajek monopole solution is found to be
asymptotically stable and causal. The so-called horizon boundary condition of
Znajek is shown to be a regularity condition at fast critical surface.
