Re: Backward Roll - Risks and benefits
Yes the way you described that Ukemi was familiar to the one that I was saying about, the only difference being that we do hit the tatami twice; once when we hit the ground at first and then when we almost "rotate" onto the other side and get up. I don't now why we hit the mat twice, all Sugawara Sensei said that the Ukemi was to protect the internal organs from shock of the fall.
However the explanation you said at the end of your post does not seem familiar. I didn't now that that was the reason why you did it, simply because Sugawara Sensei taught it as a simple Ukemi and so does my own instructor. To be done when doing techniques like Irimi Nage and Kokyo-Ho Nage, among others.
Also apparently Kobayashi Sensei taught the same thing at the UKA Summer School 2005, possibly something which Hombu Dojo is implementing. Although a suggestion but looking at the Uke in Doshus books, they never do Ushiro Kaiten Ukemi.
If I am right don't Yoshinkan teach Ushiro Kaiten Ukemi (I saw it being shown in one of the Yoshinkan DVDs), but I have never seen it being done when they are performing techniques??