(Refer to this thread).

In order to be able to keep a "SFW version" of this site, please be sure to post any NSFW content under the #nsfw tag. This means you should either reply to a thread with the #nsfw tag when posting such content or, if creating a new thread, put the #nsfw tag among the thread tags.

You may post a link to a NSFW resource in a thread without the #nsfw tag, but in this case you must highlight that the link is NSFW, for example:

{NSFW}http://link-to-nsfw-content.com/

However, it's preferrable to avoid this and just post under the appropriate threads.

Should you see NSFW content under a thread without the #nsfw tag, please report the problem to an admin so that the #nsfw tag may be added to the thread.

Following these simple rules allows the "Safe Mode" button to have some utility.