It sits in the squirmiest parts of your mind and body until (I imagine) you can't take it anymore and have to stop reading. I assume. This didn't happen to me. I, with some sort of immensely morbid fascination, couldn't stop reading the damned thing.
I just read something from an awesome author friend of mine David Owain Hughes - a new submission for Burning Willow Press, and the five chapters I read have sent me into a tailspin of, yes, pleasure. The pleasure of 'witnessing' horrid acts, but being in the MC's shoes - feeling his pleasure with him as he commits them, well if you're into BDSM that is, and understanding his pleasure.