Neil, I was always taught that your momentum cannot carry you across the baseline and I have seen the type of throw shown in the video as a rule infraction in tournament play.
I'm not sure where (or if) they define when motion is complete in any rules. I think it's more difficult to explain then the NFL's rules on the process of completing a catch. For me, I know it when I see it and is more about their intent. If they are trying to get as close as possible to the line and throw as hard as they can, they better not have their foot carry over the line at all or I'm calling it.
Thanks Jesse, that's what I thought as well.