Free writing is the rapid, nonjudgmental capturing of ideas on paper (or in your word processor) as they rise to the top of your mind.
It is a technique that fiction and nonfiction writers use when they want to generate ideas, overcome writer’s block, or become more creative.
Don’t give your brain a moment to pause or evaluate your thoughts. It doesn’t matter if you make typos or spelling mistakes while free writing — just keep getting the words out of your head and onto the blank page before the critical part of your brain has time to catch up.
Work against a limit of some sort
Limits give a writer confines within which to work. You can use a word count (the higher the better), a deadline, or a timer.
From ‘This Simple Writing Technique Will Help You Overcome the Inertia of Perfectionism‘ by Bryan Collins