Obviously it's not "moving" in any set direction or this thread wouldn't
exist, but that doesn't change the fact that we perceive movement,
which is in fact the basis of the question itself.
To me it looks like the sides are moving upward while the top and bottom are both moving to the left. Though if I focus, I can just see it as flashing...
No, the border is in Place A in the first frame, and Place B in the second,
then Place A again in the third and Place B again in the fourth. There are
two places for the border to be in and that is the "2" to which Primus is
referring. The only reason there are 4 different frames is because Zelda
didn't want the heart to be blinking as fast as the border, so she made
the heart on in 2 of the frames and off in the other 2.