I finally found a video (of another cat) doing something similar to what Balloon does. We call it her "scoot" but for the most part, it seems to be called "sideways cat" or "sideways crawl". I've never seen another cat do this, but Furball actually did it one time himself, so it must be catchy.
Balloon's is more hilarious and less locomotive than this cat's movement. She only uses her front paws to dig into the carpet and move, about an inch at a time. At this point she stops, and rolls around for a second, then continues scooting. I can't get a good video though, because she's pretty self-conscious and if we move or she thinks we're looking at her, she promptly stops and runs away. I've heard cats do this when they become overly excited or playful.
p.s.: Holy pregnant dog...
does YOUR cat scoot?