  Magnetic reconnection within a highly magnetized plasma has been seen as a
viable mechanism to extract the energy from a rotating black hole, as it can
generate negative energy plasmoids in the ergoregion. For a typical accreting
black hole, the ergoregion is filled with bulk plasma plunging from the
innermost-stable-circular orbit (ISCO). In this study, we present an analytical
study of the energy extraction via magnetic reconnection process in the
plunging region. In contrast to the toroidal plasma, where the magnetic field
cannot be derived from the MHD scheme, the magnetic field in the plunging
plasma was determined by the ideal-MHD condition. We derive the global magnetic
field structure in a fast reconnection model, and we read the expressions for
the energies of plasmoids ejected from the reconnection region, for general
stationary and axisymmetric spacetimes. Then, we demonstrate the behaviors of
ejected energies varying with the reconnection locations in the Kerr spacetime,
and identify the region where a negative-energy plasmoid can be produced. We
find that for a certain magnetization there exists a critical value of the
black hole spin, beyond which the energy extraction can occur, and the energy
extraction is most efficient for the near-extreme black hole. Moreover, we
study the conditions necessary for a plasmoid with positive energy to escape to
the infinity, a crucial requirement for effective energy extractions.
Considering the escaping conditions, we provide the parameter space in the
radius-spin plane in which the energy extraction mechanism is effective.
