It consisted of two loops, about thigh length, attached at opposite sides of a back-pad. The loops went about ones knees, the back-pad behind your sacrum/low back. The pull on the loops by your knees pulled the back-pad against you, creating a secure support for your back. All you needed to do was stay balanced.
The ilio-psoas muscles perform a similar function, although attached at your groins, not at your knees. The part that pulls on your back like the back-pad (but on the inside), we call the psoas muscles; the part that pulls on your pelvis from the inside, we call the iliacus muscles. Together, they share a tendon at your groin, and so we call them the iliopsoas muscles. They span the distances between your groin on each side and your low back and between your groin and your inner pelvis on both sides. Their pull on your low back is like the pull on the back-pad, only along more of your back as high as your diaphragm; their pull on your pelvis on both inside surfaces pulls the pelvis top-forward, adding to the support of your back.
In that way, your iliopsoas muscles are like the Nada-chair. When you are sitting in a chair, your iliopsoas muscles shorten to hold you up, especially if you are sit perched on the edge of your chair (as so many do), but those muscles shorten also in those who slouch back in their chairs and hunch forward.
somatic education program to free tight psoas muscles