He's talking about how as you specialise in something you have to systematise your disparate, formulaic knowledge of the topic into a systemic understanding tha

He's talking about how as you specialise in something you have to systematise your disparate, formulaic knowledge of the topic into a systemic understanding that you can draw new insight from. Working in that field goes from applying learned techniques to having an instinctual understanding of it as a whole, and consequently being able to be flexible and creative with your approach. I think this is what happens when something "clicks" and what were previously seemingly-disconnected rules of thumb join up to make a cohesive mental model.