Can anyone tell me what happens to posts while they're being edited? Are they still visible in their previous form, or are they temporarily off-line while being revised?

Being basically html web documents the unedited posts will remain visible until they have been updated and published. When you submit your edited post there is a time lapse after which you will see the standard 'submitted' message. This lapse is the period during which your post is uploaded to the AWAD server.
Any member currently viewing your post will still see the unedited version until they reload/refresh their browser. This also applies to new posts, of course.

The only thing I don't like about the editing option on this site is that it does not update the time and date (or position on the thread) accordingly. Some form of time stamp for editing would be very useful.