Flash fiction has been defined as being stories under a certain number of words. The number of words varies. Campbell and Rogers Press (somewhat arbitrarily) defines this type of fiction as comprised of stories that are 1,200 or fewer words long.
Flash fiction stories are not vignettes; they're complete stories told in two or three (or, occasionally more) parts. Their scenes are integral to the plot, their characters are necessary and well-drawn, and there's a purpose to the tales; each has a theme, if not a moral. Many are cautionary tales, but they need not be.
Most have a beginning, a middle, and an end. Some are of literary quality; others are genre stories. Since every word counts, flash fiction authors rewrite and rewrite, eliminating unnecessary adverbs and adjectives, trimming phrases, and pruning sentences.
There is no right or wrong way to tell a story, but some ways are more effective than others. Writers may start a story with a specific situation, with a particular character, with an unusual setting, with an inspiring theme, or with a goal, a problem, or a conflict.
Lawrence Block often writes short stories (not flash fiction) that start with an insult or an outrage that prompts revenge. These stories, employing the biter-bit plot, involve an ironic reversal in which the table is turned on an aggressive, often predatory, antagonist so that the protagonist, in gaining revenge, also gains the upper hand against his or her opponent.
O. Henry's stories offer readers a twist. An initial situation suggests a certain outcome, but the end of the story offers a surprise, an unexpected resolution of the conflict that, although unanticipated, is both plausible and appropriate. For example, in “The Gift of the Magi,” Jim sells his watch to buy combs for his wife Della, while she sells her hair to buy a fob for his watch. In “The Ransom of Red Chief,” kidnappers find their hostage, a rambunctious and highly energetic boy, more than they'd bargained for and offer to pay his parents to take him off their hands. O. Henry didn't write flash fiction, either, but his technique lends itself to tales of any length, including those much shorter than his own.
Another way to generate a story's scheme is to use a two-part approach, wherein the first part implies or demands the second: problem-solution, cause-effect, victimization-revenge, lost-found, haughty-humbled.
Edgar Allan Poe defined the short story as a narrative that a reader can finish in a single sitting. While he didn't define the length of “a single sitting,” we might suppose that the length of a typical feature film, arguably about ninety minutes to two hours, would make as good a definition as any other. Research suggests that the average reading speed is about 200 words per minute, so, if a flash fiction story is 1,200 words, the average reader could complete it in about six minutes, give or take a few seconds, whereas, reading steadily for hour and a half, a reader could finish an 18,000-word short story.
Obviously, flash fiction authors have to capture their readers' attention quickly, maintain their interest, and move from the story's beginning to its end as efficiently as possible, without seeming to force the story's resolution on the reader.
Here are a few tips that can help to make this happen: