  Type Ia supernova (SN Ia) light curves have a secondary maximum that exists
in the $r$, $i$, and near-infrared filters. The secondary maximum is relatively
weak in the $r$ band, but holds the advantage that it is accessible, even at
high redshift. We used Gaussian Process fitting to parameterise the light
curves of 893 SNe Ia from the Zwicky Transient Facility's (ZTF) second data
release (DR2), and we were able to extract information about the timing and
strength of the secondary maximum. We found $>5\sigma$ correlations between the
light curve decline rate ($\Delta m_{15}(g)$) and the timing and strength of
the secondary maximum in the $r$ band. Whilst the timing of the secondary
maximum in the $i$ band also correlates with $\Delta m_{15}(g)$, the strength
of the secondary maximum in the $i$ band shows significant scatter as a
function of $\Delta m_{15}(g)$. We found that the transparency timescales of 97
per cent of our sample are consistent with double detonation models, and that
SNe Ia with small transparency timescales ($<$ 32 d) reside predominantly in
locally red environments. We measured the total ejected mass for the normal SNe
Ia in our sample using two methods, and both were consistent with medians of
$1.3\ \pm \ 0.3$ and $1.2\ \pm\ 0.2$ solar masses. We find that the strength of
the secondary maximum is a better standardisation parameter than the SALT light
curve stretch ($x_1$). Finally, we identified a spectral feature in the $r$
band as Fe II, which strengthens during the onset of the secondary maximum. The
same feature begins to strengthen at $<$ 3 d post maximum light in 91bg-like
SNe. Finally, the correlation between $x_1$ and the strength of the secondary
maximum was best fit with a broken line, with a split at $x_1^0\ =\ -0.5\ \pm\
0.2$, suggestive of the existence of two populations of SNe Ia.
