This animation expands/collapses an arc that's 3/4ths of a circle.
The next iteration begins where the previous appeared to finish.
This happens while the whole thing rotates, such that the starting point of each
expansion moves in a star-wise pattern around the circle.

See:
https://www.google.com/design/spec/components/progress-activity.html
(note: the video here demonstrating this animation is actually slightly incorrect...
we'll hopefully update it soon)