Here's my initial thoughts.
I suppose that's like asking, "How is it that we can "activate" the English language without ever having studied the proper syntactical structure of English grammar?"
The interesting thing is, we don't even think about the actual structure of our language, and we just continue to speak it, naturally, and quite effortlessly.
So how does our ability to speak English happen so naturally without first passing through our conscious understanding of grammatical structure?
I haven't the slightest idea.
Yet, even when the grammatical rules are broken (which we all do quite frequently) and even out of order, we still understand the meaning without having been taught about the structure of how we learn to speak our first language.
Additionally, as an English (language) teacher, I've learned that conscious understanding of English grammar doesn't necessarily help the language learner. I know this because most of my own students know grammar better than I do, yet their speaking skills are nothing like their grammar.
Yet I don't consciously think about my grammar when I speak, and it just comes out naturally. Perhaps this says something about how photoreading works (or vice versa).