Although freewriting can be seen as a short writing task it is an approach to impromptu writing that merits more discussion (Peter Elbow's work in this area is invaluable). In practice, freewriting is where one usually writes for a specified period of time without stopping, and without judging the quality of the writing in any way, a kind of speaking-writing. It can encourage a more unmediated way of writing, exploring ideas, thoughts, forms of expression which may be stifled in other more public writing.
'Freewriting, like speaking, often leads to digression; yet like speaking it also helps us gist and crunch...we can pause, ... take a deep breath and ask ourselves, "So what is this all about?" (Peter Elbow, 2012, Vernacular Eloquence)
Partly as a provocation, we've used freewriting regularly with science students (and with those in the humanities who might be seen as more sympathetic). For many undergraduates in the sciences, approaching a research project or longer essay can be daunting as they may not have done any extensive writing since GCSE, and a common difficulty they raise is getting started. Freewriting is a technique that can help, and one which many find useful and (sometimes) enjoyable. As much of the research writing in the sciences is co-authored, public - as opposed to private freewriting - where freewrites are exchanged and read among the group) can help students get more used to letting others read their work.
In addition to helping to make students more comfortable with the act of writing, freewriting can also lead into more crafted activities. For examples of this and a typical prompt used with medical students, see freewriting ideas (with thanks to Phyllis Creme) and Freewrite in Medicine.
You might also want to look at a freewriting app developed by Thinking Writing and Damian Hippisley here at QMUL which allows writers to do timed freewrites online. Any feedback on this app is welcome - please contact Julian Ingle.