  Using observations from the Solar Dynamics Observatory's Atmosphere Imaging
Assembly and the Ramaty High Energy Solar Spectroscopic Imager, we present
novel measurements of the shear of post-reconnection flare loops (PRFLs) in
SOL20141218T21:40 and study its evolution with respect to magnetic reconnection
and flare emission. Two quasi-parallel ribbons form adjacent to the magnetic
polarity inversion line (PIL), spreading in time first parallel to the PIL and
then mostly in a perpendicular direction. We measure magnetic reconnection rate
from the ribbon evolution, and also the shear angle of a large number of PRFLs
observed in extreme ultraviolet passbands ($\lesssim$1 MK). For the first time,
the shear angle measurements are conducted using several complementary
techniques allowing for a cross-validation of the results. In this flare, the
total reconnection rate is much enhanced before a sharp increase of the hard
X-ray emission, and the median shear decreases from 60$^\circ$-70$^\circ$ to
20$^\circ$, on a time scale of ten minutes. We find a correlation between the
shear-modulated total reconnection rate and the non-thermal electron flux.
These results confirm the strong-to-weak shear evolution suggested in previous
observational studies and reproduced in numerical models, and also confirm
that, in this flare, reconnection is not an efficient producer of energetic
non-thermal electrons during the first ten minutes when the strongly sheared
PRFLs are formed. We conclude that an intermediate shear angle, $\le 40^\circ$,
is needed for efficient particle acceleration via reconnection, and we propose
a theoretical interpretation.
