  Our recent work demonstrates a correlation between the high-velocity blue
edge, $v_{edge}$, of the iron-peak Fe/Co/Ni $H$-band emission feature and the
optical light curve shape of normal, transitional and sub-luminous type Ia
Supernovae (SNe Ia). We explain this correlation in terms of SN Ia physics.
$v_{edge}$ corresponds to the sharp transition between the complete and
incomplete silicon burning regions in the ejecta. It measures the point in
velocity space where the outer $^{56}$Ni mass fraction, $X_{\rm{Ni}}$, falls to
the order of 0.03-0.10. For a given $^{56}$Ni mass, $M(^{56}Ni)$, $v_{edge}$ is
sensitive to the specific kinetic energy $E_{\rm kin}$($M(^{56}Ni)/M_{WD}$) of
the corresponding region. Combining $v_{edge}$ with light curve parameters
(i.e., s$_{BV}$, $\Delta m_{15,s}$ in $B$ and $V$) allows us to distinguish
between explosion scenarios. The correlation between $v_{edge}$ and light-curve
shape is consistent with explosion models near the Chandrasekhar limit.
However, the available sub-$M_{Ch}$ WD explosion model based on SN 1999by
exhibits velocities which are too large to explain the observations. Finally,
the sub-luminous SN 2015bo exhibits signatures of a dynamical merger of two WDs
demonstrating diversity among explosion scenarios at the faint end of the SNe
Ia population.
