Flash (and other) video compression uses a similar algorithm to look for parts of a video that are more static. (To avoid re-sending information about a static background, for example.) I'd guess it's a modification from compression technology.

to avoid re-sending information about a static background for example. I wonder if they built theirs from scratch or just modified

They're just overlaying sprites (stars in this case) in areas of high deltas (changes). That's all.