The brain stem (back head chakra) is what it works on. I had the most dramatic experience of this a few years ago, the first using both Reiki and homeopathy.
At the time I was very suspicious of the guides, wondering if it was some kind of self-deluding illusion and barring craziness if they could be some demonic possession like on The Exorcist.:)
Apis 6 was a remedy they wanted me to hold on my hands when I slept. At some point in the night, my hand was around the back of my head.
A numbness started in the area just at my scalpline, spread up and deep into my brain which left the area cold and my body paralysed.
It lasted a half hour or less, but when I began to move I had an enormous headache for a day or two but after a brain fog I didn't know I had lifted and didn't get eyestrain headaches from computers ever after!