  We present early-time photometric and spectroscopic observations of the Type
Ia Supernova (SN Ia) 2021aefx. The early time u-band light curve shows an
excess flux when compared to normal SNe Ia. We suggest that the early-excess
blue flux may be due to a rapid change in spectral velocity in the first few
days post explosion, produced by the emission of the Ca II H&K feature passing
from the u to the B bands on the time scale of a few days. This effect could be
dominant for all SNe Ia which have broad absorption features and early-time
velocities over 25,000 km/s. It is likely to be one of the main causes of
early-excess u-band flux in SNe Ia which have early-time high-velocities. This
effect may also be dominant in the UV filters, as well as in places where the
SN spectral energy distribution is quickly rising to longer wavelengths. The
rapid change in velocity can only produce a monotonic change (in flux-space) in
the u-band. For objects which explode at lower velocities, and have a more
structured shape in the early-excess emission, there must also be an additional
parameter producing the early-time diversity. More early time observations, in
particular early spectra, are required to determine how prominent this effect
is within SNe Ia.
