His description of it is that these memories hold an electrical charge, that charge gets re-stimulated by something familiar in the environment, and causes pain in the present time.
You may want to read the books in order, it is the recommended way.
If you are only going to read one, I have given it to you.
If you really want to study the technology, start at the beginning, and go in order.
I read Dianetics because I had been steered away from it, so I started at the beginning.
It is a technical manual where SOS is more a user's manual.
It was a difficult read for me, but I persevered and have read it several times since.
I hope it benefits you as it has me.