  We investigate mechanisms of energy extraction from a rotating black hole in
terms of negative energy-at-infinity. In addition to the Penrose process
through particle fission, the Blandford-Znajek mechanism by magnetic tension,
and the magnetohydrodynamic Penrose process, we examine energy extraction from
a black hole caused by magnetic reconnection in the ergosphere. The
reconnection redistributes the angular momentum efficiently to yield the
negative energy-atinfinity. We derive a condition for the process to operate in
a simple situation, where the plasma is incompressible and the magnetic energy
is converted completely to the plasma kinetic energy locally. Astrophysical
situations of magnetic reconnection around the black holes are also discussed.
