In my experience, when I am derealised, I feel completely numb from all sensations, so much, that I need to cut myself to bleed and to feel pain, in order to feel alive. This is, in another word, self-harming, by cutting myself on the wrists and thighs. When people see others cut themselves, thinking that it must be really painful to do so, however, when I am derealised, I feel no nothing. That is why I cut, for relief and to gain that powerful thrill of pain, which the term “pain” to those who have been through Complex-PTSD would perceive differently, because pain to us is something that we get used to all the time, and it becomes our comfort zone. Thus, the opposite side of pain, would be panic zone.
This is all of a personal experience, knowing that everyone reacts differently to trauma. Some may dissociate by other means, and not necessarily the same as me. Some traumatised individuals may seek drugs and alcohol to cope with the inner pain, some may develop certain addictions to dissociate themselves and some may seek medication or medical treatments, instead of self-miltilation.