  In this paper we examine various issues closely related to the ongoing
discussion on the nature of the Blandford-Znajek mechanism of extraction of
rotational energy of black holes. In particular, we show that switch-on and
switch-off shocks are allowed by the shock equations of relativistic MHD and
have similar properties to their Newtonian counterparts. Just like in Newtonian
MHD they are limits of fast and slow shock solutions and as such they may be
classified as weakly evolutionary shocks. The analysis of Punsly's MHD
waveguide problem shows that its solution cannot have the form of a traveling
step wave and that both fast and Alfv\'en waves are essential for generating
the flow in the guide. Causality considerations are used to argue that the
Blandford-Znajek perturbative solution is in conflict with the membrane
paradigm. An alternative interpretation is presented according to which the
role of an effective unipolar inductor in the Blandford-Znajek mechanism is
played by the ergospheric region of a rotating black hole. Various implications
of this are discussed.

Astrophysics