Electric Fish Dance... that's cute. My fiancee jsut calls it seizures. I like your wife's name better.
Anyway, it's a submissive gesture. It's always been non-horizontal in my experience... my theory is that it's the equivalent to us laying prone. In that position, we really can't do much, and are therefor allowing the harasser to do whatever they please. It's the fish's way of saying "I give up."
But that's just my theory. I haven't exactly been in this hobby for long.