What is the point of the masturbation scene in Donnie Darko?

Honestly I think it has to do with what Donnie's personality is like. He often seems very cynical and void of emotion, except when his sister says he's a dick. He usually just observes things like a zombie of some sort. I think what the scene represents is another side of Donnie that most people wouldn't see normally. The psychiatrist is trying to break through like most normal doctors and understand Donnie. With the hypnosis and him coming out like that I feel it was almost a warning to her that if you want to dig deep into him, you may not like what you see. That I also feel is a metaphor throughout the movie. We can see it in the guru, the gym teacher, and other characters. They are one way when we look at them, but when you did deeper it may not be something we wanted to know in the first place.