Rephrasing paragraph from story

I've recently read great story: https://en.zity.biz/index.php?mx=docs;ox=showpart;idp=10171

I have an issue with understanding something from this text. Here is that paragraph:

"... I stripped, then stood there bashfully. She briefly smiled at me. Then, she took a kitchen chair, and placed it on its side on the towels.

"Now Steven, I want you to kneel on your knees and chest, but over the back of the chair."

This meant that my thighs were vertical, and my back was strongly arched.

"You see Steven, some of my patients try to curl their bottom inwards, away from me. The chair-back prevents this. Enjoy it if you can. Be proud to expose your anus to the world!" ..."

I can't figure out the position on the chair that was orderd by Steven's aunt. Someone (especially native speaker) can rephrasing it for me?

I wish I could draw a picture.

The chair is lying on its side atop some towels, on either its two left or two right legs. The same edge of the chair's seat and back are on the towels, too.

So the back of the chair is now as high as it ordinarily is wide. A person could bend over it with their thighs vertical. Because they're being ordered to place their chest on the floor, on the other side of the chair's back, this will make their back arch and their buttocks spread open.

Does this help it make sense to you?

Thank you for your explanation. It's now everything clear for me.