It's when your brain can't handle any more of the trauma and it tries to help your body and mind survive so your mind goes somewhere else it considers safer. For example, my PTSD is the result of childhood sexual abuse... incest. When things were being done to me rather than just go crazy my mind would take me away. While I was still very young it would take me to my grandmother's house and, as I grew older it many times took me to books or stories I had read. You literally feel transported, your mind "dis" "associates" from your body. It usually happens involuntarily but I found once I realized it could happen I could sometimes consciously will it to happen. It's a bit like meditation, the more you work at it the easier it becomes. I know you have lots of questions and I'm happy to share what I've learned and, the more you understand how something works the easier it becomes to control the controllable parts of it.