The video that is primarily streaming there is http://www.youtube.com/watch?v=ZU2kyr9jRkg , which is completely blank. But you an go to the URLs tab (http://www.amara.org/en/videos/UnjvMd1yQrXx/info/sandbox/?tab=urls) to add the URL of another video, and make it primary in order to caption/subtitle it.
It would be great if some of you could try it and post feedback about it in the comments (http://www.amara.org/en/videos/UnjvMd1yQrXx/info/sandbox/?tab=comments). Please remember to download subtitles/captions you make there if you want to keep them, as they will get deleted after a week or two.
It's not a real sandbox, in that you still need to have an Amara ID and be logged with it in order to use it. Also, subtitles/captions will be manually, rather than automatically, deleted. Nevertheless, it might be of use for people who want to try Amara, but are uneasy with having links to their first attempts in their Amara profiles, which get indexed by search engines. If a profile says: "[Amara ID] edited English subtitles for Sandbox", for instance, it does not matter so much: sandboxes are meant for exploring tools.