  We explore the presence of torques at the inner edges of geometrically-thin
black hole accretion disks using 3-dimensional magnetohydrodynamic (MHD)
simulations in a pseudo-Newtonian potential. By varying the saturation level of
the magnetorotational instability that leads to angular momentum transport, we
show that the dynamics of gas inside the radius of marginal stability varies
depending upon the magnetic field strength just outside that radius. Weak
fields are unable to causally connect material within the plunging region to
the rest of the disk, and zero torque is an approximately correct boundary
condition at the radius of marginal stability. Stronger fields, which we obtain
artificially but which may occur physically within more complete disk models,
are able to couple at least some parts of the plunging region to the rest of
the disk. In this case, angular momentum (and implicitly energy) is extracted
from the material in the plunging region. Furthermore, the magnetic coupling to
the plunging region can be highly time dependent with large fluctuations in the
torque at the radius of marginal stability. This implies varying accretion
efficiencies, both across systems and within a given system at different times.
The results suggest a possible link between changes in X-ray and outflow
activity, with both being driven by transitions between weak and strong field
states.
