  We critically examine the theory of disk locking, which assumes that the
angular momentum deposited on an accreting protostar is exactly removed by
torques carried along magnetic field lines connecting the star to the disk. In
this letter, we consider that the differential rotation between the star and
disk naturally leads to an opening (i.e., disconnecting) of the magnetic field
between the two. We find that this significantly reduces the spin-down torque
on the star by the disk. Thus, disk-locking cannot account for the slow
rotation ($\sim$10% of breakup speed) observed in several systems and for which
the model was originally developed.
