Derailing tends to happen on Fora and Blogs the most, but it's generally something that can happen anywhere there is a discussion of any sort. It is simply the unwanted and forced change of a subject to one that the derailer wishes to discuss, or one with which the derailer wishes to start a Flame War or a flare of Internet Backdraft in hopes of getting the original topic forgotten.
Derailing differs from simple Trolling in that where the Troll wants to start a fight or cause damage, consequences be damned, the consequence of the topic being changed is the goal of someone engaged in derailing. This often can make methods of combating trolls (such as the application of whole thread locking and deletion or the banning of trolls and others behaving badly) worthless at best and in some cases extremely counterproductive in an attempt to deal with a derailing attempt.
Discussions that are Serious Business or reference Serious Business tend to attract derailing the most, especially anything referencing a Real Life issue or incident or discussing Unfortunate Implications. Common methods of derailing often include victim blaming, various forms of Ad Hominem including but not limited to poisoning the well and direct Ad Hominem, raising an entirely separate concern that is at most only loosely related to the subject under discussion (e.g. a tax rant on a thread discussing response times for the fire department or a rant about fathers' rights in a feminist issues discussion), Misplaced Nationalism, and outright Trolling.
Finally, derailing must be intentional, if it is not intentional, it is simply the Left Fielder. This is the "forced" part of it: if, say, a general discussion on pit bulls turns into a fight about how dangerous pit bulls are, it wasn't derailed, since that was an arguable natural outcome of the discussion, even if a bad or unwanted one. A discussion about training pit bulls turning into a raging Flame War over whether one can breed a pit bull with a Chihuahua due to one poster posting graphic pictures of the mating, however, is very much likely so due to derailing. As another hypothetical example, mentioning Adolf Hitler in a discussion about neo-Nazis is not derailing. Comparing your opponents in a discussion about an MMORPG to Adolf Hitler, on the other hand, is.
Compare GIFT and Troll, compare Garbage Post Kid which is occasionally an extreme example of derailing, and compare Godwin's Law, which is almost always an instant derail of any discussion in which it is an Invoked Trope. See Serious Business, Flame War, and Internet Backdraft. Derailing is a favorite tactic of The Shill and those engaged in the use of Astroturf and Polish the Turd, and it is actually advice sometimes given to deal with an Internet Counterattack. Related to Griefer and Troll in that the derailer, the griefer, and the troll all seek disruption, albeit, as mentioned, in different contexts for different goals.
(The best way to tell the difference is to ask oneself what the goal seems to be. If there seems to be no intent at all, it is the Left Fielder. If the intent is enraging everyone in range, it's likely a Troll, or if in a game, a Griefer. If the intent is to shut someone or everyone up or change all discussion to the new topic, it is likely Derailing.)
- Derailing for Dummies is a sarcastic Parody "guide for derailers" in discussions about social justice issues. It lists some of the most common derailing tactics in action for those interested in keeping discussions/communities on topic and stopping derail attempts.
- Pretty much anything that appears on the Flame War, Internet Backdraft, or Internet Counterattack pages will illustrate derailing in action quite well.
- Or done more benevolently for a thread that's heating up to or embroiled in flames. This is only as successful as the ratio of posters willing to stop taking themselves seriously.
- A small scale meme similar to this is often used as an anti-Trolling tactic on GameFAQs. If a poster makes an obvious troll topic, a topic that is still a sore spot for a community, or a topic that has been overdone, another poster will say something along the lines of "This topic sucks. Its now about trains." Cue the other posters discussing everything they know about the railroad industry.
- A humorous example emerged from The Jedi Council Forums. An otherwise regular forum debate brought up Waru, and suddenly within hours, a board-wide Church was formed.
- As a combination of this and Astroturf, the "Internet Water Army," which uses trolling and Derailing as an Astroturf tactic. Note that while the Water Army may be exclusive to China and/or Chinese issues, any Real Life Mega Corp, political party, or even religion with enough devotees or money could manage the same thing.