The essence of the Blandford–Znajek process

In a spacetime perspective, dynamics of magnetic field lines of force-free electromagnetic fields can be rewritten into a quite similar form for dynamics of strings, that is, dynamics of "field sheets." Using this formalism, we explicitly show that the field sheets of stationary and axisymmetric force-free electromagnetic fields have identical intrinsic properties to the world sheets of rigidly rotating Nambu-Goto strings. Thus, we conclude that the Blandford-Znajek process is kinematically identical to an energy-extraction mechanism by the Nambu-Goto string with an effective magnetic tension.