  We discuss the underlying relativistic physics which causes neutron stars to
compress and collapse in close binary systems as has recently been observed in
numerical (3+1) dimensional general relativistic hydrodynamic simulations. We
show that compression is driven by velocity-dependent relativistic hydrodynamic
terms which increase the self gravity of the stars. They also produce fluid
motion with respect to the corotating frame of the binary. We present numerical
and analytic results which confirm that such terms are insignificant for
uniform translation or when the hydrodynamics is constrained to rigid
corotation.
  However, when the hydrodynamics is unconstrained, the neutron star fluid
relaxes to a compressed nonsynchronized state of almost no net intrinsic spin
with respect to a distant observer.
  We also show that tidal decompression effects are much less than the
velocity-dependent compression terms. We discuss why several recent attempts to
analyze this effect with constrained hydrodynamics or an analysis of tidal
forces do not observe compression. We argue that an independent test of this
must include unconstrained relativistic hydrodynamics to sufficiently high
order that all relevant velocity-dependent terms and their possible
cancellations are included.
