It just happens. If your browser for some reason navigates away from the page, if you go back the previous window it will not be lost. Even if I've typed something in a window once it's 'autosaved' and remove it if I try to navigate away from the page it will warn me.
I've always been careful to keep long posts saved to my clip boardt hough, I think the feature might possibly be of use to some people. I hit Cntrl-A + Cntrl-C plus end which will select copy and go back to the end of any active text box.