  Single holes confined in semiconductor quantum dots are a promising platform
for spin qubit technology, due to the electrical tunability of the $g$-factor
of holes. However, the underlying mechanisms that enable electric spin control
remain unclear due to the complexity of hole spin states. Here, we study the
underlying hole spin physics of the first hole in a silicon planar MOS quantum
dot. We show that non-uniform electrode-induced strain produces nanometre-scale
variations in the HH-LH splitting. Importantly, we find that this
\RR{non-uniform strain causes} the HH-LH splitting to vary by up to 50\% across
the active region of the quantum dot. We show that local electric fields can be
used to displace the hole relative to the non-uniform strain profile, allowing
a new mechanism for electric modulation of the hole g-tensor. Using this
mechanism we demonstrate tuning of the hole $g$-factor by up to 500\%. In
addition, we observe a \RR{potential} sweet spot where
d$g_{(1\overline{1}0)}$/d$V$ = 0, offering a configuration to suppress spin
decoherence caused by electrical noise. These results open a path towards a
previously unexplored technology: engineering of \RR{non-uniform} strains to
optimise spin-based devices.
