  In a semiconductor spin qubit with sizable spin-orbit coupling, coherent spin
rotations can be driven by a resonant gate-voltage modulation. Recently, we
have exploited this opportunity in the experimental demonstration of a hole
spin qubit in a silicon device. Here we investigate the underlying physical
mechanisms by measuring the full angular dependence of the Rabi frequency as
well as the gate-voltage dependence and anisotropy of the hole $g$-factors. We
show that a $g$-matrix formalism can simultaneously capture and discriminate
the contributions of two mechanisms so far independently discussed in the
literature: one associated with the modulation of the $g$ factors, and
measurable by Zeeman energy spectroscopy, the other not. Our approach has a
general validity and can be applied to the analysis of other types of
spin-orbit qubits.
