Recent discussion in Horror thread compelled me to re-watch "Martyrs" and "Cannibal Holocaust". This forced me to adjust my understanding of what I consider disturbing. Some of my reactions to those films included "nice CGI" or "I'm not a vegetarian". You can't go much further is respect to violence than that, but that is pretty much the only aspect of those films that can disturb.
I can misquote that, but Mark Kermode asked William Friedkin why "The Exorcist" was accepted by public and "The Devils" not, even though films share many similarities. Answer was: almost every aspect of Russell's film was excessive. Now I think he was right and this can be applied to other disturbing films, examples to follow.
"Irreversible" is attacking audience from so many angles: camerawork, low quality and clarity of images, nauseating music and sound, structure of the film, improvised dialogue that is sometimes very trivial (even stupid). "Possession" is also excessive: naked sets (almost no extras), acting inspired by Grotowski and weird behaviors, dialogues/monologues full of dogmatics, doppelgangers and so on.
My point is that you can be directly hit, even though it is single shot limited to one aspect of the film e.g. violence. Yet it is probably even harder to detach yourself, if you are in the middle of crossfire, being attacked by something else than just violence.