  We develop a new model within which the radius-dependent transition of the
subhalo inner spins with respect to the cosmic web and the variation of the
transition threshold radius ($r_{\rm th}$) with subhalo mass ($M_{\rm vir}$),
smoothing scale ($r_{f}$), and redshift ($z$) can be coherently explained. The
key tenet of this model is that the competition between the pressure effect of
the inner mass and the compression effect of the local tidal field determines
which principal direction of the tidal field the inner spins are aligned with.
If the former predominates, then only the tidal torques turn on, resulting in
the alignments of the inner spins with the intermediate principal axes of the
tidal field. Otherwise, the subhalo spins acquire a tendency to be aligned with
the shortest axes of the subhalo shapes, which is in the major principal
directions of the tidal field. Quantifying the two effects in terms of the
densities, we make a purely analytical prediction for $r_{\rm th}(M_{\rm vir},
z, r_{f})$. Testing this model against the numerical results from a
high-resolution dark matter only N-body simulation in the redshift range of
$0\le z\le 3$ on the galactic mass scale of $11.8\le \log[M_{\rm
vir}/(h^{-1}M_{\odot})]\le 12.6$ for two different cases of $r_{f}/(h^{-1}{\rm
Mpc})=0.5$ and $1$, we find excellent agreements of the model predictions with
the numerical results. It is also shown that this model naturally predicts the
alignments between the inner spins of the present subhalos with the principal
axes of the high-$z$ tidal field at the progenitors' locations.

Cosmology and Nongalactic Astrophysics;Astrophysics of Galaxies