To understand what these things are, one must first get out of the "restraint" mindset that is commonly shown in horror movies. These are not restraints. They are used to move people that cannot move themselves. Think about it this way, you are a ninety pound nurst trying to move three hundred pounds of dead weight. Now tell me, do you need help?
The world of special needs also has quite a few devices to this day that look like something used to restrain someone in a horror flick. Take a look a a standing frame someday :)

Yipes, sorry I am posting so much. I looked at the photograph again and it is upside down. The hole in the middle is about where a person's crotch would be. A leg would be on the longer part to either side. There really is no good head support on these things, aside from the whole wrapping up feeling of the sling. all four sides and a few other places attach to the metal part that lifts and lowers the person.

Ah yes, it hooks onto a metal device too. There is a lever that you push up and down that raises it. A release slowly lets it back down.

That is a sling for a Hoyer lift. They come in all sizes and are used in moving and lifting people who do not have the ability to do that themselves. Welcome to my world, the world of a special needs mother. I have a Hoyer in my home to help with the lifting.