Magnetic Extraction of Spin Energy from a Black Hole
  Numerous variations have been proposed on the original suggestion by Blandford and Znajek that magnetic fields could be used to extract rotational energy from black holes. A new categorization of these variations is proposed so that they may be considered in a systematic way. "Black hole spindown" is defined more precisely, distinguishing decrease in the spin parameter a/M from decreases in angular momentum a and rotational kinetic energy, M - M_i. Several key physical questions are raised: Can the "stretched horizon" of a black hole communicate with the outside world? Do accretion disks bring any net magnetic flux to the black holes at their centers? Is the magnetic field adjacent to a black hole force-free everywhere? 
Julian H. Krolik