For some mysterious reason, it helps many people to write down that wrong word rather than to stop their writing and search mentally for a better one. It's true that freewriting is good for unskilled students, but we find that skilled, experienced writers and teachers get the most out of freewriting. It's an exercise whose payoff increases with the experience of the writer. You can't predict how freewriting will work for you. Some people start off with coherent freewriting, but as they use it, they gradually drift into freewrting that's more jumpy and surprising.
Some people start off jumpy.
It is important to give yourself permission for this and not measure freewriting by the quality of what you turn out. Trust the Process. New Investigations of Freewriting. Southern Illinois UP, 1991. Writing with Power .