what's it called when people hang from nails on back?

Answer 1

Flesh Hook Suspension. Read about these rituals performed all over the world at Source. The history of piercing flesh then pulling against it or hanging from it dates back thousands of years, in such diverse cultures as India, the Middle East, and North America.

Answer 2

It's a form of self-flagellation or self-mortification, usually done as part of some religious ritual or as an act of penance for sins that were committed.

Answer 3

Masochism?