I remember practicing something similar to this when I did some Aikikai training under Clyde Takeguchi of Capital Aikikai in Silver Spring, MD. He would take us through techniques that ended up in a back stretch most times. The techniques used were mainly Sokumen, Irimi Nage and Shi Ho Nage, with Uke relaxing into the back stretch.
From my personal experience in doing Shiatsu, Makko Ho and Qigong, I realised that these stretches in Aikido had some close relation to meridian theory. In many cases an elongation of the Conception Vessel meridian, and those of the upper chest area (Lung, Heart etc.) was revealed in the case of the back stretches, allowing blockages to be cleared through correct breathing.
In Shodokan we do a cool down stretch (I've seen this done in Judo too) where 2 people stand back to back and one person pulls the other's arms forward and above his head and sinks his weight, flattening his back. This way the other person is lying atop the other's back, stretched out along his partner's spine. This is great for releasing blockages in the meridians along the back, especially the Bladder and Kidney meridians in my experience.
Not pretending to be a professional at this, but these have been a few observations that I have made during training. Hope it helps.