  Magnetic fields lines are trapped in black hole event horizons by accreting
plasma. If the trapped field lines are lightly loaded with plasma, then their
motion is controlled by their footpoints on the horizon and thus by the spin of
the black hole. In this paper, we investigate the boundary layer between
lightly loaded polar field lines and a dense, equatorial accretion flow. We
present an analytic model for aligned prograde and retrograde accretion systems
and argue that there is significant shear across this "jet-disk boundary" at
most radii for all black hole spins. Specializing to retrograde aligned
accretion, where the model predicts the strongest shear, we show numerically
that the jet-disk boundary is unstable. The resulting mixing layer episodically
loads plasma onto trapped field lines where it is heated, forced to rotate with
the hole, and permitted to escape outward into the jet. In one case we follow
the mass loading in detail using Lagrangian tracer particles and find a
time-averaged mass-loading rate ~ 0.01 Mdot.
