Thanks guys for the replies! I think Mike got what I'm trying to say and I'll try to do what you wrote and hopefully with some positive results. Yeah, it looks very similar to a forward roll but actually it's quite very different!
In what way is it very different? Because in the videos (and for that matter, in Mike's description, although he describes rolling rather high up on the back with less of a diagonal than usual) it looks like one of those funny forward rolls people often do from, e.g., tenshinage or something where you can twist your upper body enough to start rolling even though your lower body is still pointing the 'wrong' way.