By cutting themselves, they are able to refocus their attention on the pain of the cut and for a short while, experience relief from the rape trauma.
So, a person who dissociates may not remember something that happened to them that was very painful because through the process of dissociation they were able to store that painful memory in a section of their mind that the rest of themselves doesn't know how to find.
Feelings can also be dissociated or detached from the events that provoked them such as occurs in Depersonalization Disorder.
Sometimes an aspect of a person's chaotic internal experience is that feelings and thoughts build up a subjective state of tension or arousal that motivates people to do something to take action to reduce that tension or arousal.
Cutting or burning one's self can, apparently, serve such a tension-reducing function. I read an account of a young woman who was consumed with suicidal thoughts which pushed her towards a compulsive feeling that she should end her life.
Some of them will self-injure so as to generate a strong sensation that is capable of bringing them back to feeling something again. Not all numb dissociated people will use physical pain to "return"; some will use other strong sensations such as are afforded by drugs or sex, etc. Sometimes, people who self-harm are not skilled at using language to describe their inner experiences.