How does the placebo/nocebo effect work exactly?

Please do not answer with ''belief'' or ''expectations'' as that does nothing to answer the underlying mechanism.

I just think the placebo/nocebo effect is very silly. It implies the brain goes ''I think I've been exposed to caffeine, therefore I must cause symptoms of caffeine consumption!''
That is alike to a person setting himself/herself on fire because he/she believed his/her clothing had been tainted with white phosphorous. It makes absolutely no sense at all.

I tried to convince my brain I just took sleeping pills, but my brain for some reason is not producing sleeping pills inside of my blood. Yet there was a story a while back about a man who consumed fake pills assuming they were anti-depressants. He consumed 20+ of them in a suicide attempt and although the pills were harmless, his blood pressure dropped perilously low. This case has never been explained, I am inviting skeptics to debunk this.