Ergospheres
The region between the inner and outer event horizons of a rotating black hole is called the
ergosphere.
It is termed this because it is possible to derive energy
from it ("ergo" refers to energy).
A body can enter this region and break into two pieces
in such a way that one part falls inside the inner event
horizon but the other escapes the black hole with more energy than the original body had.
By this means, energy and angular momentum
can be extracted from the black hole, which slows its rotation a tiny
amount in response. It is also possible to extract angular momentum by scattering
electromagnetic waves in the ergosphere.