I worked on this ages ago :). It's replaying the last minute's edits, and since all of the editing tools just file a changeset (like a commit) all at once, it has to reconstruct what it looked like to draw those features. That timing is just based on a fixed count of seconds it should take to draw a line, a point, etc. The code's here:

https://github.com/osmlab/show-me-the-way/blob/gh-pages/js/s...