I have been fisting ass since 1982, and I had an expert bottom teach me precisely how to do it, what to feel for, etc. I'm not a proctologist, so I am sadly uninformed regarding proper nomenclature, but I understand what the "second sphincter" is, where it's located, etc.
It's a second ring of muscle that, essentially, closes the rectum from the colon beyond. In most guys (and women, presumably), this occurs anywhere from five to seven inches past the anus. Some bottoms, whether because they are less experienced or because their "second sphincter" is especially tight (or close), need special care in opening them up thoroughly and enough for something larger than a five-six inch cock. Some prefer that this be done digitally, others prefer toys, still others will eventually relax, and with enough time and patience will open up completely on their own with just your cock. There are no hard and fast rules here: everybody's different.
I'm sure that an unprepared ass subjected to unfettered pounding could suffer extreme damage, but I find it unlikely in the extreme that any porn producer would risk such harm to his/her talent (and the possible liability such an injury might provoke) to allow such things to be routine. Always remember that porn is very carefully edited to maximize the fantasy at the expense of the actual reality of what's going on: this is as much for gay as it is for straight porn. What appears to be an unprepared ass being "assaulted" by a "massive" dick is so grossly irresponsible as to (practically) never actually happen.
That's not to say never: never say never. But the chance of anyone documenting such abuse ranges from highly improbable to nil: there are simply too many well-paid professionals on a porn set to allow that to happen. "Amateur" productions and companies specializing in especially brutal porn might not be quite as cautious, but for such a thing to happen in a "mainstream" shoot just doesn't make sense, and remember that most "amateur" or "brutal" porn is as carefully lit, rehearsed and prepared for as any other type: it only appears less so.
It should be said that extreme assplay of any kind is not to be attempted haphazardly. Ideally both participants should have sufficient experience to know not just their own, but each other's bodies, too. I have the good sense and judgment to only play sober (myself) and with a partner who's both willing and experienced. I have no interest whatsoever in "trying something new" with a novice to such extreme play. Handled with patience, care and constant intercommunication, all of which is essential, extreme assplay, whether with toys or fists/arms should not pose any danger to either party: anything else is grossly irresponsible and should be subject to both civil and criminal prosecution, as it cannot be consensual.
