kay, in addition to what was said, let me just describe to you how I see Pain as a way to release from Genjutsu. When you're inside a genjutsu, you're inside a realm of illusion, and, especially when the genjutsu alters the majority of your senses, it gets harder to distinguish what's real and what's not. Although some genjutsus give the illusion of pain, actual physical pain is almost like an infusion of reality that snaps you out of the realm of illusion. It translates in a spike and even a loss of concentration, but it is mainly that abrupt sense of reality that pushes the genjutsu away. Of course, this is also why different levels of pain will only break different levels of genjutsu. The lowest would be clicking your finger (which is a pain you're used to) That would be able to counter, say, D ranks. Stepping um the game, weak physical attacks would break C, freestyle and low ranked taijutsu or bitting your inside cheek would break say B rank, strong taijutsu stabbing yourself, breaking smaller bones, low level burning/voltage would break A rank. EIG/Sage level taijutsu, breaking bones, deep stabs, high voltage/burning would break any other genjutsu. These are not set in stone, just some sort of reference table for you to follow.
Usually, the most used method is stabbing your thigh with a kunai. Should break anything up to A rank, deepen the cut, should break S ranks. You can do it yourself, or use a jutsu for the same effect (Normally I'd suggest that, since it could be handseal-less, and you might even use it to attack your opponent).
I think that's pretty much all there is to know about Pain releases. One thing though, many of these methods, even if insufficient to break a genjutsu, will be translated as physical reactions under the ilusion, which can be used as signs you're in a genjutsu (For example, if a fire jutsu is incoming, you might be able to sense the heat increasing but see no fireball, so you add two plus two and come to the conclusion you're in a genjutsu. Likewise, say someone slaps you. It is not enough to break you out, but, you feel the slap without explanation and it might be just enough for you to realize and use kai. (Because sometimes it's not about you being unable to use Kai, but just being unable to realize you're in a genjutsu).
>_> Told you Gen wasn't my strong point lol. Sorry for taking so long.