The core thesis of the book (per Claude) is: "Traumatic experiences become encoded in the body's nervous system, muscles, and organs, not just in conscious memory."
The perfect exposition of the concept is downthread, in a comment by 'neom, which I will excerpt: "[The acupuncturist] moved the needle, [I did] more crying, deeper, deeper crying, he kept moving the needle till I thought all the needles would burst out of me from how deeply I wanted to cry but he told me not to be scared and I thought I was going to die. Anyway, he left me alone in that room for about 35 minutes while I wailed, I mean, awkwardly wailed. After everything started to calm inside me, I slowly started to be able to think again, and the thought that was there was the memory of the guy who sexually abused me when I was a kid, moving his hand off my hip." (Thank you for sharing, neom.)
I've had two similar experiences myself in the last year. I haven't read the book and don't know how this subject shows up in the scientific literature, but the proprioceptive experience leaves zero room for doubt about what is happening.
People say lots of dumb and wrong things about trauma. Maybe the book contains some of that, but its titular observation is fascinating, true, and maybe even useful!
I'll close with a related theory of mine, also derived through proprioception: one of the functions of the full-body sob is to reorganize muscular patterns which share an origin with (are identical to?) the emotional origin of the tears.