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Abstract 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>∼</mml:mo> <mml:mn>0.01</mml:mn> <mml:mspace width="0.25em"/> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:math> .