I think this is a pretty good description of how, and why, people dissociate during trauma, and how it is actually helpful.
According to Mauro, it’s during the “freeze response” that you can experience disconnect. Because there aren’t any other options available, you essentially sever contact between your brain and body in order to survive the experience. This is a similar survival response to a mouse “playing dead” when caught by a cat to increase its chances of getting out of there alive.
While dissociation is a helpful strategy at the time, it can also arise long after the trauma is over, causing problems in your daily life. Dissociation might occur when you encounter a situation or object that reminds your nervous system — consciously or subconsciously — of the trauma.
Of course, using this same defense mechanism in later life can be a problem, and when your brain has used this as a sort of “last resort” choice to survive, that might also mean that your memory of the event isn’t what others would like it to be. That’s OK, it helped you survive, that is the important thing. The rest is irrelevant if you don’t survive, so we are glad about that. Once you’ve survived, we can work on the rest of it.
Read more about the what, why, and how of dissociation at the link below.