  Thermonuclear burning on the surface of a neutron star causes the expansion
of a thin outer layer of the star, $\Delta R(t)$. The layer rotates slower than
the star due to angular momentum conservation. The shear between the star and
the layer acts to twist the star's dipole magnetic field giving at first a
trailing spiral field. The twist of the field acts in turn to `torque up' the
layer increasing its specific angular momentum. As the layer cools and
contracts, its excess specific angular momentum causes it to {\it rotate
faster} than the star which gives a leading spiral magnetic field. The process
repeats, giving rise to torsional oscillations. We derive equations for the
angular velocity and magnetic field of the layer taking into account the
diffusivity and viscosity which are probably due to turbulence. The magnetic
field causes a nonuniformity of the star's photosphere (at the top of the
heated layer), and this gives rise to the observed X-ray oscillations. The fact
that the layer periodically rotates faster than the star means that the X-ray
oscillation frequency may ``overshoot'' the star's rotation frequency.
Comparison of the theory is made with observations of Chakrabarty et al. (2003)
of an X-ray burst of SAX J1808.4-3658.
