In our particular flavor of Tomiki aikido, we have a set of movements called releases that sorta act as beginnings. Then we have the basic kata techniques in Junana and Owaza that act as endings. You can, to a large extent piece together most any technique in aikido by starting with one of the releases and ending with one of the kata finishes.
So, where does ushirowaza fit in with respect to the releases? Easy. Most of the ushirowaza are about the same things as releases #1, 2, or 5.
In fact, at the beginning of ABG 2010, I posed the question, "In our system of aikido, what is our first exposure to ushirowaza?" Folks mumbled and stumbled around and finally came up with, "Koryu Dai San." Well, actually it is at white belt, not quite the first day of practice but soon thereafter - Release #5 is usually introduced to beginners by saying, tori is trying to do #1 but uke is trying to turn the corner and take tori's back, so tori steps under uke's arm.
It turns out that Release #1 is very hard to stop tori from doing. There are several kata variations (like Yonkata #7) that illustrate a release#1 from an extreme ushiro position - deep in the hole. But every so often uke grabs your back with sufficient vigor that you can't get #1 to work, so what are the backup plans for release #1? Release #2 and Release #5.