When writing something that matters, there are at minimum two steps involved.
The first step is the act of dumping down all thoughts and informations in one place. In this step, the brain needs to be as open as possible to better find connections between ideas. Accordingly, the analytical part of the brain that deals with the correctness of the language and the structure and overall quality of the final product needs to be turned off. The brain needs to explore as far as it can, unhindered from rules.
The second step is sculpting. This is when the analytical part of the brain is allowed to wake up and do its job: shape the language, cut the unnecessary weight, alter the flow, challenge the logic.