Guru Siyag has said it over and over again. He speaks of 7 layers of the body. Now he also insists that you repeat the mantra round the clock to progress on this path, but how many people do it? And why is it important? It’s very simple. Although first you might start out by chanting the mantra somewhat physically (a bit of involuntary tongue movement) and somewhat mentally (thinking about the mantra as opposed to just intoning it, eventually the supramental bodies (to put it in Sri  Aurobindo Ghose terminology) will take over the chanting.

And then that happens, you will experience again and again the tendency to perceive and japa jap using the mental and sub-mental layers of your being.

And so basically you just keep improving your ability to chant the mantra using the supramental koshas.

Good luck.