PROIRON. Balance Cushion

You can sit at your desk using your lower abdominal core muscles, as if you were sitting on a balance ball. I used this to help with back pain. The photo shows it with only half the air filled. If you add more air, it becomes like just the seating part of a balance cushion, and your body bounces easily.
In my case, I filled it with about half the air, tucked the middle part of the cushion between the backrest and seat of the chair, and sat on it by bending it. It felt like only the upper half of my thighs were on the cushion. This created a forward-sloping angle, and the air in the cushion moved softly when I engaged my lower abs.
I bought two, one for school and one for home, but I didn't use this home one because I was studying on a balance ball. No instruction manual or packaging included.