Did you know this?
During the 1972 Winter Olympics in Sapporo, the skater Janet Lynn fell during her routine and managed to smile and flash the V sign, which really struck a chord with the Japanese about good sportsmanship, doing your best and smiling through adversity. The media loved her spirit and girls all over Japan copied it.
If you ask young girls why they do it, though, few if any can tell you any reason at all.