The “command” words are words mainly used in BDSM to show control. Hence the dom and sub, (it really is a dumb idea, but also funny) it’s also supposed to emulate hypnosis which is also a hard kink where you program trigger words into an individual, in most cases the sub, to feel and get a certain way with the trigger (command) words. In real life this can happen, but it’s not as exaggerated as in these types of stories. IRL the one programmed can snap out of it or ignore it if they want to or are in an uncomfortable setting. It’s all about trust which is also what these stories are about. Trusting the one who is in control.
I hope that helps a little, but the short of it is it’s hypnosis with commanding words.

Like do they have to just say the word itself for it to be considered a command? Or do they have some kind of psychic power that does so? Why is it only certain words that seem to have a innuendo to it, that it's considered a command? Like this concept makes no sense to me cause like I can't for the life of me take these people seriously when they just use everday words like that. So in case if the dom says 'Die', would the sub just go along with it? Or what?