  If a supermassive black hole has some material orbiting around it at close to
its innermost stable circular orbit (ISCO), then, when it plunges into a second
supermassive black hole, the orbiting material has a velocity dispersion of
order of speed of light about the orbital velocity of its host black hole. It
becomes plausible that some of the orbiting material will be "catapulted" to
the negative-energy ergosphere orbits of the second black hole at the plunge.
This may provide an astrophysically plausible way to extract energy from the
black hole, originally suggested by Penrose.
