I used this same technique to induce lucid dreams when I was younger. My theory for why it worked was that one part of the brain was sending a 'roll over' signal to another part of the brain in order to check if it was ready to enter the next stage of sleep. By purposely ignoring that signal and remaining still, I was able to fall directly into a dream while still conscious.

I'm sure there was a bit more to it than that. I also kept a rigorous dream journal, and had several other rituals and practices that I've since forgotten. But lying still was definitely a big part of it.