  AM CVn binaries are a class of ultracompact, hydrogen-deficient binaries,
each consisting of a white dwarf accreting helium-dominated material from a
degenerate or semi-degenerate donor star. Of the 56 known systems, only
Gaia14aae undergoes complete eclipses of its central white dwarf, allowing the
parameters of its stellar components to be tightly constrained. Here, we
present phase-resolved optical spectroscopy of Gaia14aae. We use the spectra to
test the assumption that the narrow emission feature known as the `central
spike' traces the motion of the central white dwarf. We measure a central spike
velocity amplitude of $13.8 \pm 3.2$ km/s, which agrees at the 1 $\sigma$ level
with the predicted value of $17.6 \pm 1.0$ km/s based on eclipse-derived system
parameters. The orbital phase offset of the central spike from its expected
position is $4 \pm 15$ $^\circ$, consistent with 0 $^\circ$. Doppler maps of
the He I lines in Gaia14aae show two accretion disc bright spots, as seen in
many AM CVn systems. The formation mechanism for the second spot remains
unclear. We detect no hydrogen in the system, but we estimate a 3 $\sigma$
limit on H$\alpha$ emission with an equivalent width of -1.14 \AA. Our
detection of nitrogen and oxygen with no corresponding detection of carbon, in
conjunction with evidence from recent studies, mildly favours a formation
channel in which Gaia14aae is descended from a cataclysmic variable with a
significantly evolved donor.
