How does fear paralysis occur? (This question is in relation to this question here)

How does fear paralysis occur? What is the mechanism that makes this happen in brain?
Taken from here

The Fear Paralysis Reflex begins to function very early after
conception and should normally be integrated before birth.  It can be
seen in the womb as movement of the head, neck and body in response to
threat.
It is sometimes classified as a withdrawal reflex rather than
a primitive reflex. If this reflex is retained after birth, it can be
characterised by withdrawal, reticence at being involved in anything
new, fear of different circumstances, and is often described as the
“scaredy cat” child who bears the brunt of teasing by normally
adventurous children.
This behaviour appears to be
due to the reflex’s involvement with the parasympathetic nervous
system.  Most of us are familiar with the “fight or flight” adrenalin
rush of the sympathetic nervous system, however the FPR taps into the
opposing system urging the body to “eat and stay”.  The
parasympathetic nervous system is intimately involved with the vagus
nerve.  This nerve comes directly from the brain to aid the organs.
It bypasses the spinal cord, so in the case of spinal injury we are
still able to digest our food.  The vagus nerve may be mechanically
trapped in the skull, chest, abdomen or neck.  Release of this nerve
entrapment corrects one physical factor that contributes to the
retention of the FPR.