  In this paper we review some aspects of the theory of magnetic threaded
disks. We discuss in particular the equations that determine the position of
the inner disk boundary by using different prescriptions for the neutron
star-accretion disk interaction. We apply the results to several accretion
powered X-ray pulsars that showed both quasi-periodic oscillations in their
X-ray flux and spin-up/spin-down torque reversals. Under the hypothesis that
the beat-frequency model is applicable to the quasi-periodic oscillations, we
show that these sources provide an excellent opportunity to test models of the
disk-magnetosphere interaction. A comparison is carried out between the
magnetospheric radius obtained with all the prescriptions used in threaded disk
models; this shows that none of those prescriptions is able to reproduce the
combination of quasi-periodic oscillations and torque behaviour observed for
different X-ray luminosity levels in the X-ray pulsars in the present sample.
  This suggests that the problem of accretion disk threading by stellar
magnetic field is still lacking a comprehensive solution. We discuss briefly an
outline of possible future developments in this field.
