Scary thing to witness this!
My husband also "does" this.
He roll's his eyes very fast, it looks very scary, he is unable to move en freezes. He can still hear me talking during this, but more like i am standing in a different room.
My husband thinks he did this when he was a kid, during the abuse. Some small children that are abused also invent ways to dissociate, the don't do it on purpose, the have to, to survive.
I think you are right Roe, its is some kind of dissociative flashback.
Though a powerfull one when he is totally out.
You could let him check it out medically, just to be sure its cause is psychological. To be safe and sure.
Recovery from severe trauma does a lot of things to the brain.
There actually is some sort of braindamage.
I read somewhere that the right part of the brain is sometimes taking over completely during flashbacks and the left side is shut down so there is no access to emotional understanding or memory.
I don't think you are supposed to do anything Roe, Just be there.
I think its wise to be there and stay present during this.
There is also a lot of information on the internet on how to support a partner during a flash-back.
On the website of Jim Hopper i think as well. Some good stuff on yourtube as well.
Next to that, your own feelings are important to! I admire your strenght Roe! And you are not alone take care of yourself. Maybe find yourself someone to talk with, a therapist or something and ofcourse you can vent here!