A lot of people start to have a flashback and their body can't handle the intensity and dissociate in order to protect themselves, the way they did when the abuse actually happens. Something triggers the flashback and the flashback triggers the dissociation. If you want to get technical, no they may not be happening simultaneously, but they do happen together often, side by side.

Everyone has their own experience and while it's important to have terms and diagnoses to go by, the DSM is limited to terms and definitions and greatly discounts actual human experiences (emotions, individual expression, etc.) - at least in my opinion.

Despite the technicality of the terms, I think most people can relate to the situation being described. Developing and inner sense of safety and learning how to ground yourself, along with different techniques to learn to tolerate the more extreme emotions can be really helpful. It can make the flashbacks seem less real, and more manageable over time.