  Kilohertz-scale quasi-periodic oscillations (kHz QPOs) are a distinct feature
of the variability of neutron star low-mass X-ray binaries. Among all the
variability modes, they are especially interesting as a probe for the innermost
parts of the accretion flow, including the accretion boundary layer (BL) on the
surface of the neutron star. All the existing models of kHz QPOs explain only
part of their rich phenomenology. Here, we show that some of their properties
may be explained by a very simple model of the BL that is spun up by accreting
rapidly rotating matter from the disk and spun down by the interaction with the
neutron star. In particular, if the characteristic time scales for the mass and
the angular momentum transfer from the BL to the star are of the same order of
magnitude, our model naturally reproduces the so-called parallel tracks effect,
when the QPO frequency is correlated with luminosity at time scales of hours
but becomes uncorrelated at time scales of days. The closeness of the two time
scales responsible for mass and angular momentum exchange between the BL and
the star is an expected outcome of the radial structure of the BL.
