  We present TESS photometry of the asynchronous polar BY Cam, which undergoes
a beat-cycle between the 199.384-min white dwarf (WD) spin period and the
201.244-min orbital period. This results in changes in the flow of matter onto
the WD. The TESS light curve covers 92% of the beat cycle once and 71% of the
beat cycle twice. The strongest photometric signal, at 197.560-min, is ascribed
to a side-band period. During times of light-curve stability, the photometry
modulates at the spin frequency, supporting our WD spin-period identification.
Both one-pole and two-pole accretion configurations repeat from one beat cycle
to the next with clear and repeatable beat-phase dependent intensity
variations. To explain these, we propose the operation of a magnetic valve at
L1. The magnetic valve modulates the mass-transfer rate, as evidenced by a
factor of 5 variation in orbital-averaged intensity, over the course of the
beat cycle in a repeatable manner. The accretion stream threading distance from
the WD is also modulated at the beat-period, because of the variation of the WD
magnetic field with respect to the stream and because of changes in the mass
transfer rate due to the operation of the magnetic valve. Changes in the
threading distance result in significant shifts in the position of accreting
spots around the beat cycle. As a consequence, only the faintest photometric
minima allow for an accurate ephemeris determination. Three regions on the
white dwarf appear to receive most of the accretion flow, suggestive of a
complex WD magnetic field.
