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 aM _ { 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 _ { H } c ^ { 2 } .
This is larger than \approx 0.09 M _ { H } c ^ { 2 } which can be extracted by the Blandford-Znajek mechanism .