Watching the clip from Mask of Satan got me thinking about different types of body horror and what makes some types more uncomfortable to watch than others. What really triggers my squeamishness is any type of violence directed specifically at the face. Watching the woman get branded didn't make me anywhere near as uncomfortable as the face mask scene. In fact, if they would have put the woman in a body cast full of spikes but left her face alone, I wouldn't have found that as disturbing, even though it could arguably have been stretched out into a slower and more painful death on camera.
I can't really pinpoint why this is. I suppose this could be turned into some psychological argument about vanity, but I don't think that's the driving force. It may be because the head is the most vulnerable part of the human body. We have a nice helmet of bone to protect our brain (which, actually, can still be crushed or pierced pretty easily), but our eyes and mouths are kind of like having internal organs on the outside of our bodies. In Mask of Satan during the point-of-view shot when the mask is being applied, the viewer's eyes are communicating to the brain images of the spiked instrument that is about to pierce them and ultimately kill the victim. We normally protect our eyes by throwing our arms up in front of them or moving our head or blinking. In this case the victim is bound, her head is being held in place, and her eyelids are no match for spikes. Placed in her position by the point-of-view shot, the viewer fully realizes how easily his or her own existence can be ended and what a miracle it is that we can live for decades when one well-aimed blow to the head could have taken any of us out at any time in the blink of an eye.
Along with the eyes, the mouth seems equally vulnerable. I don't know the science behind nerve ending distribution in the body, but there is something about dental torture that makes me much more uncomfortable than watching someone have their fingers cut off, etc. Marathon Man springs to mind....
Interesting that this scene, like Mask of Satan, ends with a point-of-view shot seen through the victim's eyes. The shot begins with the drill, then the dentist's right eye, and then the light--the same order that someone in a dentist chair would focus on individual aspects of the procedure.