  When some magnetic field lines connect a Kerr black hole with a disk rotating
around it, energy and angular momentum are transferred between them. If the
black hole rotates faster than the disk, $ca/GM_H>0.36$ for a thin Keplerian
disk, then energy and angular momentum are extracted from the black hole and
transferred to the disk ($M_H$ is the mass and $a M_H$ is the angular momentum
of the black hole). This way the energy originating in the black hole may be
radiated away by the disk.
  The total amount of energy that can be extracted from the black hole spun
down from $ca/GM_H = 0.998$ to $ca/GM_H = 0.36$ by a thin Keplerian disk is
$\approx 0.15 M_Hc^2$. This is larger than $\approx 0.09 M_Hc^2$ which can be
extracted by the Blandford-Znajek mechanism.
