Catharsis in Humiliation?

I have been researching the produced screenplays written by Erin Cressida Wilson because she is supposed to have had a hand in the screenplay of "Stoker" according to the NY Times. Wilson's movies all have similar themes, exploring the fringes of human sexuality - I was not wholly unfamiliar with her work, having seen "Fur: an Imaginary Portrait of Diane Arbus." and I think being the only one that liked it. However, her first movie, "Secretary" was supposed to have been a big hit in the indie world - and apparently I missed it when it debuted in 2002. All that being said, I watched it today and DO NOT get it.
       Here's what I don't get... the reveling in humiliation. Not wholly unfamiliar with the BDSM lifestyle although I don't really subscribe to it myself, I understand the psychology of relinquishing control and the need to be in control and the currency of trust in a loving relationship and to a degree I understand, though do no prefer, the concept of stretching the bodies limits and boundaries of pain and pleasure. However, I do NOT understand what catharsis a stable, emotionally healthy individual would get from real humiliation, either giving or receiving. As a matter of fact, I don't see what catharsis is achieved for an unhealthy individual.

Trust? um... where and why? And Trust is supposed to be the point of BDSM relationships... so again, WHAT? As a dominate person in such a relationship, if your job is to see to the welfare of the submissive, I am going to assume that means their mental and emotional well being as well - Where does humiliation play a part? What is achieved or gained? I wish someone would explain this to me.