Through the time I've been learning aikido, I observed my fellow aikidoka's ukemi and through various demonstration that there is seemingly a "new" type of ushiro ukemi that is not widely taught, learned or known, or at least this is true in the country I'm practicing in. I can identify three types of ushiro ukemi - the first is probably the oldest which is the reverse of mae ukemi, second is when you bent down one of your leg and make a v-shape with your two legs, the third and last which is the one I identify as "new" is sort of an "advanced" and more "dynamic" version of the second version of ushiro ukemi. I have added youtube videos incase my explanation if confusing.
(2:10, they call it yoko ukemi)
(1:34 Tada's uke performing the "advanced" ushiro ukemi)
The major difference I can identify comparing this ukemi to the 2nd ukemi is that you don't bend down one leg when initiating the ukemi, but you keep that leg straight and push backwards with that leg(correct me if I'm wrong) and therefore there is more momentum and "dynamic". I find that the advantage of this type of ukemi it that in situations when nage executes the technique "strongly", the force can be absorbed and used in propelling the ukemi instead of a receiving a strong impact when you bent on leg such as in the 2nd ukemi i explained. Both create a v-shape with your legs, but one seem more fluid and soft, as well as I see many are able to return to a standing position after the execution of the ukemi.
One fellow aikidoka in my dojo is able to perform this type of ukemi very well, he sort of pioneered this type of ukemi into our dojo, and many are trying to imitate and learn this kind of ukemi. I noticed this type of ukemi is quite popular in USAF videos i watch in youtube, such as yamada's and donovan sensei's ukes. It seemingly is also used and taught in japan as well, as when I was praciticng in hombu dojo some sensei's would try to teach this in beginner class. I observed alot of ukes used in aikido demonstration also uses this especially in the IAF.
I'm very passionate in learning this ukemi! personally i find it very elegant and beautiful :P and I would like to seek training advices regarding this ukemi. Like i mentioned, this ukemi is not too technically understood in my dojo so It is hard for us to learn it other then trial and error. If anyone is willing to give advices regarding this version of ushiro ukemi, i would greatly appreciate it