A few things I've noticed about free writing, in no particular order:
The first three minutes are almost always garbage. You know this going in. You sit down, you start moving the pen or the keys, and what comes out is some version of "I don't know what to write, I'm tired, the coffee isn't working." That part isn't the point but it's also not skippable. It's the hallway you walk through to get to the room.
Peter Elbow, who basically coined the term "free writing" in Writing Without Teachers back in 1973, had one rule that mattered more than all the others: don't stop. He didn't care what you wrote. He cared that you kept going. The pen doesn't lift. The fingers don't pause. Fifty years later I still think that's the best single piece of free writing tips anyone's offered.
Speed matters more than you'd think. When you write fast, you outrun the part of your brain that wants to sound smart.
I've noticed that the things I write during free writing that embarrass me a little are usually the things worth keeping. There's something about the slight cringe that signals honesty.
Natalie Goldberg talks in Writing Down the Bones about writing practice the way a musician talks about scales. You don't play scales because scales are beautiful. You play them so your fingers know where to go when the song starts. Free writing is scales for your sentences.
Most people think they need more ideas. In my experience, they need fewer ideas and more willingness to stay with one until it gets interesting.
There's a strange thing that happens around minute seven or eight, where the writing shifts. The sentences get less performative. You stop trying to say something and start actually saying something. I'm not sure why this works on a timer like that, almost like clockwork, but I've seen it happen too many times to ignore it.
Ray Bradbury used to say, "Quantity produces quality. If you only write a few things, you're doomed." He wrote every single day, sometimes thousands of words before breakfast, and he treated that daily volume the way a prospector treats dirt: you move enough of it and eventually you find something.
Free writing doesn't care about your outline.