  The effect of mass and energy loading on the efficiency at which energy can
be extracted magnetically from a Kerr black hole is explored, using a
semi-analytic, ideal MHD model that incorporates plasma injection on magnetic
field lines. We find a critical load below which the specific energy of the
plasma inflowing into the black hole is negative, and above which it is
positive, and identify two types of flows with distinct properties; at
sub-critical loads a magnetic outflow is launched from the ergosphere, owing to
extraction of the black hole spin energy, as originally proposed by Blandford
and Znajek. At super-critical loads the structure of the flow depends on the
details of the injection process. In cases where the injected plasma is
relativistically hot, a pressure driven, double trans-magnetosonic flow is
launched from a stagnation point located outside the ergosphere, between the
inner and outer light cylinders. Some fraction of the energy deposited in the
magnetosphere is then absorbed by the black hole and the rest emerges at
infinity in the form of a relativistic outflow. When the injected plasma is
cold an outflow may not form at all. We discuss the implications of our results
to gamma ray bursts and active galactic nuclei.
