  Although the occurrence of steady supercritical disk accretion onto a black
hole has been speculated about since the 1970s, it has not been accurately
verified so far. For the first time, we previously demonstrated it through
two-dimensional, long-term radiation-hydrodynamic simulations. To clarify why
this accretion is possible, we quantitatively investigate the dynamics of a
simulated supercritical accretion flow with a mass accretion rate of ~10^2
L_E/c^2 (with L_E and c being, respectively, the Eddington luminosity and the
speed of light). We confirm two important mechanisms underlying supercritical
disk accretion flow, as previously claimed, one of which is the radiation
anisotropy arising from the anisotropic density distribution of very optically
thick material. We qualitatively show that despite a very large radiation
energy density, E_0>10^2L_E/(4 pi r^2 c) (with r being the distance from the
black hole), the radiative flux F_0 cE_0/tau could be small due to a large
optical depth, typically tau 10^3, in the disk. Another mechanism is photon
trapping, quantified by vE_0, where v is the flow velocity. With a large |v|
and E_0, this term significantly reduces the radiative flux and even makes it
negative (inward) at r<70r_S, where r_S is the Schwarzschild radius. Due to the
combination of these effects, the radiative force in the direction along the
disk plane is largely attenuated so that the gravitational force barely exceeds
the sum of the radiative force and the centrifugal force. As a result, matter
can slowly fall onto the central black hole mainly along the disk plane with
velocity much less than the free-fall velocity, even though the disk luminosity
exceeds the Eddington luminosity. Along the disk rotation axis, in contrast,
the strong radiative force drives strong gas outflows.
