To lend some perspective. I've encountered this eel enema thing on more than one occasion in Japanese porn. In the most extreme case I've seen, the girl was in apparent extreme despair and other enema ingredients were involved as well as extreme bondage. She may have done this voluntarily and it may, in part, be acting, but it sure as hell didn't seem so. I don't think you can fake such extreme distress. I couldn't finish watching it, coz it was just too realistic and I was literally on the verge of an emotional meltdown. In many cases you will see the girl at the end laughing and apparently "none the worse for wear".
For the most part in extreme fetish Japanese porn the girls are apparently involved voluntarily and technically of their own volition. I don't really know what the dealio is, but it seems they see such treatment as acceptable with some element of submissiveness & humiliation. There are some seriously deranged people out there. - I don't consider this to fall under the category of safe role playing.
I gave up dl'ing Japanese porn that has no English title as it's just too harrowing overall.
I dunno; I'm just a soft porn kinda guy - I like my porn all chicks [with pretty lingeries] and no "dicks" : )