  The unique binary AR Scorpii consists of an asynchronously rotating,
magnetized white dwarf (WD) that interacts with its red-dwarf companion to
produce a large-amplitude, highly coherent pulsation every 1.97 minutes. Over
the course of two years, we obtained thirty-nine hours of time-resolved,
optical photometry of AR Sco at a typical cadence of 5 seconds to study this
pulsation. We find that it undergoes significant changes across the binary
orbital period and that its amplitude, phase, and waveform all vary as a
function of orbital phase. We show that these variations can be explained by
constructive and destructive interference between two periodic, double-peaked
signals: the spin-orbit beat pulse, and a weaker WD spin pulse. Modelling of
the light curve indicates that in the optical, the amplitude of the primary
spin pulse is 50% of the primary beat amplitude, while the secondary maxima of
the beat and spin pulses have similar amplitudes. Finally, we use our timings
of the beat pulses to confirm the presence of the disputed spin-down of the WD.
We measure a beat-frequency derivative of -5.14(32) x 10^-17 Hz/s and show that
this is attributable to the spin-down of the WD. This value is approximately
twice as large as the estimate from Marsh et al. (2016) but is nevertheless
consistent with the constraints established in Potter & Buckley (2018). Our
precise measurement of the spin-down rate confirms that the decaying rotational
energy of the magnetized white dwarf is sufficient to power the excess
electromagnetic radiation emitted by the binary.
