On the Blandford - Znajek mechanism of the energy loss of a rotating   black hole
  The Grad--Shafranov approach to the problem of the structure of the black hole magnetosphere is discussed. For the double transonic flow, the number of boundary conditions in the pair creation region is shown to be sufficient to determine not only the longitudinal electric current, but also the angular velocity of a flow as a solution to a problem. As a result, the energy loss is determined by the physical parameters at the particle creation region rather than the "boundary conditions" at the event horizon. 
V.S.Beskin, I.V.Kuznetsova