CHARMSPEAKING

So, I was reading a book when I had a sudden thought. How can the characters in books charm speak? Is it like brainwashing them? Or is it trying to make them feel guilty not listening to them? It is completely illogical. I mean, sure, most things in Rick Riordian`s books are illogical, but this really gets to me. Also, is charmspeaking having to do with the voice, of the person the sound comes out from? For example, if I had a robot copy Piper Mclean`s voice charmspeaking, then broadcasted it to the world, would they obey it? Or, if someone who could charmspeak wrote an order, like 'eat pickles', would that work as charmwriting? Someone answer my questions, because I have no idea what Riordian was up to when he wrote the idea of charmspeaking. And how come the word charmspeaking shows up red underlined on the text but not the title? Thank you.