I am reading about HQET in Grozin's book http://www.amazon.es/Effective-Theory-Springer-Tracts-Physics/dp/3540206922. While constructing the Lagrangian he first consider the usual QCD Lagrangian with only one heavy quark and many lights. He then says that if we consider characteristic momenta of the heavy quark smaller than its mass we cann simplify the energy momentum dispersion relation to

$$E=m$$

and that as a result the bilinear part of the heavy quark Lagrangian can be taken to be

$$\bar{Q}(i\gamma^o\partial_0-m)Q$$

he then states that we can use two component spinors to describe spin instead of four component ones. Can somebody give me a justification for this last step? don't we lose degrees of freedom this way?