Typing is the tip of the muscle memory iceberg. Coding in flow happens when your fingers are so familiar with your word and letter patterns that they type for you without you thinking about it. Your brain stays in the mental space of thinking about code while your fingers produce it.

This happens at multiple levels. We learn certain patterns of code -- creating object literals, file headers and so on -- that we barely notice what we're typing, they just kind of flow out.

A mark of a Senior developer, beyond collaboration and communication skills, is the amount of code they don't think about but just flows out subconsciously. It leaves their conscious minds more open to contemplate why they're writing what they're writing.