An 80-minute documentary about Kink.com, the world's leading purveyor of BDSM videos (bondage, dominance and submission, sado-masochism, etc.), Kink is intended for general audiences unfamiliar with this subset of the estimated $13 billion-a-year internet porn industry. But that doesn't make it easy to watch. We meet film directors, assistants and actors, who are not allowed to perform unless they enjoy the depicted activities — unlike other kinds of porn, faking of pleasure is not allowed. Almost all of the participants espouse an admirable live-and-let-live perspective. But the on-set footage shows more than some of us will ever want to see, including extended torture scenes (elements of which are faked) and brief snippets of full-on pornography. It's not for the faint of heart. Directed by Cristina Voros and produced by James Franco, Kink doesn't attempt to explain why people are aroused by graphic scenes of brutality — at least in BDSM's most extreme forms — beyond what the interview subjects are willing to reveal of themselves. By the end of the film, it seems that some things are better left unexplained.