  Supra-arcade downflows (SADs) are dark voids descending through plasma above
the post-flare arcade. Although they are generally viewed as byproducts of
flare reconnections in the corona, the nature of SADs is under debate. Here we
investigated six distinct episodes of SADs observed in the post-maximum phase
of an M-class flare of April 11, 2013. Differential emission measure analysis
revealed that SAD cases occurring close to the flare maximum contain an
enhanced hot plasma component at 5--7~MK whereas those occurring later
exhibited a depression in hot plasma at 7--12~MK compared to the ambient
supra-arcade plasma. On-disk location of the flare enabled us to examine in
detail the interaction of SADs with the post-flare arcade, whose effects
include 1) transverse oscillations of period $\sim\,$160~s in the supra-arcade
rays in the wake of voids, 2) footpoint brightening in 1700{~\AA} whose peak is
delayed by 22-46~s with respect to the SAD's arrival at the top of the arcade,
and 3) EUV intensity perturbations expanding and propagating with a speed
$\sim\,$400 km~s$^{-1}$. On the other hand, due to line-of-sight confusion in
the optically thin corona, the ribbon enhancement following the interaction
produces an illusion of plasma rebound at the top of the arcade, where the
interaction fails to yield significant plasma heating. These effects indicate
that the interaction mainly generates MHD waves propagating toward the surface,
which may further produce quasi-periodic brightening at flare ribbons,
therefore contributing to quasi-periodic flare gradual phase emission in EUV.
