A big spoiler:
Aron amputates his arm; it is extremely graphic. He first breaks one bone by hitting it against the side of the canyon. He really struggles with the second bone, which is broken by doing the same thing. He is already in lots of pain, but then starts slicing his arm with a razor. Blood and muscle tissue goes all around his arm; there is a very quick shot of him ripping out an artery. After that is the nerve, which looks a lot like dental floss thin spaghetti and when he touches it, a sound effect seeming like one from the board game "Operation" is heard. This is for effect and would not happen in real life. When he pulls it out, he screams in pain. This is the part of the scene widely considered the most unsettling and it is notable for reportedly making people pass out while watching it in the movie theater. He is free after around 10 more seconds of cutting.