This article is from the soc.history.what-if FAQ, by Anthony Mayer firstname.lastname@example.org with numerous contributions by others.
Newsgroup shorthand for complete disbelief in some suggested historical
reasoning: "alien space bats would be a more believable explanation."
For a while, they were being pressed into service for questions about the
effects of impossible events actually happening, but their primary use
remains for attacks on unrealism in timelines (Alison Brooks' page, see
Question 19, gives the canonical example). They are still occasionally
invoked as a quirky deus ex machina for impossible AHs, because no one has
come up with anything better. It should be noted that the ASBs' creator,
Alison Brooks (now sadly deceased), regretted the use of the ASBs as a
supernatural agency, preferring to restrict them to rhetoric.