  Most black hole candidates exhibit characteristic power-law like hard X-ray
emission above $\sim$ 10 keV. In the {\em high state}, in which 2 -- 10 keV
luminosity is relatively high, the energy index of the hard X-ray emission is
usually greater than 1 --- typically $\sim 1.5$. On the other hand, in the {\em
low state}, the hard X-ray energy index is 0.3 -- 0.9. In this paper, we
suggest that this difference of the hard X-ray spectral slopes may be due to
two different Comptonization mechanisms. We propose that, in the high state,
the hard component is governed by the Comptonization due to the bulk motion of
the almost freely falling (convergent accretion) flow close to the black hole,
rather than thermal Comptonization. The spectral slope of the hard component is
insensitive to the disk accretion rate governing the soft component, hence is
nearly invariant in spite of the soft component variations. The power-law
component due to the bulk motion Comptonization has a sharp cut-off at around
the electron rest mass energy, which is consistent with high energy
observations of the high state. In the low state, the spectrum is formed due to
thermal Comptonization of the low-frequency disk radiation by a sub-Keplerian
component (possibly undergoing a centrifugally-supported shock) which is
originated from the Keplerian disk. In the limit of low disk accretion rate,
the power law index is uniquely determined by the mass accretion rate of the
sub-Keplerian component.
