  Near maximum brightness, the spectra of Type Ia supernovae (SNe Ia) present
typical absorption features of Silicon II observed at roughly 6100A and 5750A.
The 2-D distribution of the pseudo-equivalent widths (pEWs) of these features
is a useful tool for classifying SNe Ia spectra (Branch plot). Comparing the
observed distribution of SNe on the Branch plot to results of simulated
explosion models, we find that 1-D models fail to cover most of the
distribution. In contrast, we find that TARDIS radiative transfer simulations
of the WD head-on collision models along different lines of sight almost fully
cover the distribution. We use several simplified approaches to explain this
result. We perform order-of-magnitude analysis and model the opacity of the Si
lines using LTE and NLTE approximations. Introducing a simple toy model of
spectral feature formation, we show that the pEW is a good tracer for the
extent of the absorption region in the ejecta. Using radiative transfer
simulations of synthetic SNe ejecta, we reproduce the observed Branch plot
distribution by varying the luminosity of the SN and the Si density profile of
the ejecta. We deduce that the success of the collision model in covering the
Branch plot is a result of its asymmetry, which allows for a significant range
of Si density profiles along different viewing angles, uncorrelated with a
range of $^{56}$Ni yields that cover the observed range of SNe Ia luminosity.
We use our results to explain the shape and boundaries of the Branch plot
distribution.
