Extracting Energy
Roger Penrose proposed a mechanism to extract energy from a rotating black hole. Since the ergosphere contains rotational energy, it is theoretically possible to send a particle into the ergosphere, split it into two, and have one part fall into the black hole while the other escapes with more energy than the original particle had.
Negative Energy Orbits
Inside the ergosphere, it is possible for a particle to have negative total energy relative to an observer at infinity. If one fragment of the split particle falls into the black hole on such a trajectory, it reduces the black hole's mass and angular momentum. The other fragment escapes with energy greater than the input, effectively stealing rotational energy from the black hole.
Efficiency Limit
The Penrose process is remarkably efficient compared to nuclear fusion. While fusion converts about 0.7% of mass to energy, the Penrose process can convert up to 20.7% of the infalling mass into extracted energy for a maximally rotating black hole.