This article was fairly good, all things considered, but you need some background in order to get the most out of it. The kind of magic King does not eventually perform (goetic magic) falls under what's called "ceremonial magic". Key to this sort of magic is time/resource-consuming preparatory work (which besides constructing expensive seals, may include fasting / ritual cleansing).
When King says at the end, "the key unlocks the door but you are the one who has to walk through it", he's partly referring to the preparatory but also imprinting process. The fact that one has to spend a lot of precious personal time and money (if one follows the directions of the grimoire to the letter) means that one becomes invested in the process with many changes happening in the psychological realm. Think of the whole preparatory process as ritual, acting it out strengthens and imprints the work. Or stated another way, one is constructing a new model of reality and simultaneously imprinting it.
When the time for the actual conjuration comes, which could be years after one first sets foot upon the path, the mind has been adequatly prepared to open the doors to the unconscious. Again, King seems to possess an understanding of this when he talks about the brain being the medium for the work. So, ultimately one can say that there's nothing supernatural here and people who have stayed at the supernatural level are either charlatans or deluded fools.
There's a good movie one can watch about ceremonial magic, called A Dark Song. Even though it does have some elements that can be superficially described as supernatural, it's -smartly- playing with the interpretation and does not rigidly impose a supernatural causality.