The Penrose process is a process theorised by Roger Penrose wherein energy can be extracted from a rotating black hole.
That extraction is made possible because the rotational energy of the black hole is located not inside the event horizon of the black hole,
but on the outside of it in a region of the Kerr spacetime called the ergosphere,
a region in which a particle is necessarily propelled in locomotive concurrence with the rotating spacetime.
All objects in the ergosphere become dragged by a rotating spacetime.
In the process,
a lump of matter enters into the ergosphere of the black hole,
and once it enters the ergosphere,
it is forcibly split into two parts. For example,
the matter might be made of two parts that separate by firing an explosive or rocket which pushes its halves apart.
The momentum of the two pieces of matter when they separate can be arranged so that one piece escapes from the black hole,
whilst the other falls past the event horizon into the black hole.
With careful arrangement,
the escaping piece of matter can be made to have greater mass-energy than the original piece of matter,
and the infalling piece has negative mass-energy.
Although momentum is conserved the effect is that more energy can be extracted than was originally provided,
the difference being provided by the black hole itself. In summary,
the process results in a slight decrease in the angular momentum of the black hole,
which corresponds to a transference of energy to the matter.
The momentum lost is converted to energy extracted.
The maximum amount of energy gain possible for a single particle via this process is 20.7%.
The process obeys the laws of black hole mechanics.
A consequence of these laws is that if the process is performed repeatedly,
the black hole can eventually lose all of its angular momentum, becoming non-rotating, i.e. a Schwarzschild black hole.
In this case the theoretical maximum energy that can be extracted from a black hole is 29% its original mass.
Larger efficiencies are possible for charged rotating black holes. 
