This explanation is based on the Polyvagal theory which says that there are 3 branches of the nervous system:
Dorsal Vagal, which is responsible for sleep and freeze. The way this system responds to threat is to freeze – “play dead” more or less – slow everything way down.
Sympathetic, which is responsible for activity, getting things done. The way this system responds to thread is to fight or flee – ramp everything up.
“Social Engagement nervous system”, which includes the Ventral Vagal system plus several muscle groups in the head and neck, is responsible for pleasant, authentic and collaborative interactions between humans (all mammals actually).
And this explanation is also based on the understanding of how traumatic memories or programs get implanted into our body systems:
When there’s a threat, and we can’t negotiate out of it using our social engagement system, then we go into fight or flight mode. But if for whatever reason or set of reasons, we can’t successfully fight or flee (we might not even try because we are so outgunned, as if I am a young child under threat by a caregiver) then at some point our system engages the Freeze response. However, since the threat is still real, our sympathetic fight/flight system is still running full tilt. This would allow us to quickly respond in an act of attack or escape if an opportunity presented itself. So the Sympathetic system is running full tilt and the Freeze is also running full tilt.
Now, if there is never a satisfactory and comprehensive enough resolution to this traumatic experience, wherein I am seen and heard and can feel and release all of the fear, helplessness, terror, anger, and overwhelm that showed up in me, if that safe resolution does not happen, then we get the ingredients for PTSD, or generally for unresolved trauma. In this case, the reality of the threat continues to live in the body even when the threat is far away geographically and/or far gone in the past. That means that inside my system, my flight/flight system continues to run full tilt and my freeze system also continues to run full tilt.
For some people this Fight/Flight AND Freeze state is a constant backdrop. For others, the body mind system might function pretty well for months or years, and then some trigger will awaken the unconscious memory of that threat, and the Fight/Flight AND Freeze reactions will suddenly both come on board.
And now we need to look at what these nervous system branches do – what are their jobs? Here I’m weak on the details, but I can share the big picture. To do its jobs of ramping up for action in case of threat, as well as other jobs, the sympathetic nervous system sends commands to the endocrine system, which then sends various chemicals throughout the body to do various tasks.
And to do its jobs of slowing down the body to preserve energy, as well as other jobs, the dorsal vagal nervous system sends commands to the endocrine system, which then sends various chemicals throughout the body to do various tasks.
The most common metaphor here is that the sympathetic system is the gas pedal and the parasympathetic system (including both Freeze and Social Engagement) is the brake. Normally, these two systems should work in harmony with each other. When Sympathetic is in high gear, Freeze will be in low gear. When Freeze is in high gear, Sympathetic will be in low gear. (At this point, the gas vs brake metaphor doesn’t quite work because you would not have the gas on and a little bit of brake, or vice versa. But it’s just a metaphor to provide some illustration.)
When both Sympathetic and Freeze are running full tilt at the same time, they are sending chemical instructions to the body that are at cross purposes. The metaphor here could be in the middle of winter, turning on the furnace and opening all the windows. There can be direct results of these contradictory chemical instructions, one system might call for increased blood flow and the other might be constricting the blood vessels. And there can be indirect results, which could be the depletion and exhaustion of various body systems due to working so hard at cross purposes with another body system. This is the big picture pattern of what happens when there is a significant unresolved trauma complex running in a person’s body-mind system. And the specific biological details of how that face-off between Sympathetic and Freeze plays out in each person’s body is what leads to the details of what syndrome or syndromes show up symptomatically in that body system: Chronic Fatigue, Migraine, Environmental Sensitivity, Auto-Immune, and so on.
Note that this is not to say that all syndromal symptoms are caused by nervous system dysregulation. But it is a simplified explanation of the mechanism by which the combination of Sympathetic and Freeze can be the cause of these kinds of symptom patterns, or can exacerbate such patterns.