Since flash fiction came of age, I don't know, in the '60s? I've studied the form for insights into distinguishing features. Flash fiction forums and venues, respectively, each expresses some sense of what they believe exemplifies the form. Flash Fiction Online, for example, wants short fiction between five hundred and a thousand words. They also want short fiction with dynamic, interesting characters, plots, and settings.
Lengths differ, though shorter than two thousand words in general, some as few as two hundred words. Dynamic characters are more in demand than overt plots and settings emphases, though some organizing principle is expected if a conventional plot structure does not organize a piece. Settings are challenging for short forms. Some telling setting details are expected; otherwise, the piece will feel like it's coming from a disembodied mind. Setting details evoke creative activity and engage creative minds.
I first started on this path when a writing mentor of some repute claimed flash fiction retains the raw quality of the original flash of inspiration. This, too, is challenging. The instinct to rework into a polished piece that diminishes some of a piece's strengths is strong. After reworking for polish, though, restoring the rawness of the flash of inspiration is as challenging to do as an accomplished opera singer relearning to sing like a novice. An influential exercise, though.
This past year I've dug ever deeper into the flash fiction form. Is it different from longer prose? How? Why? You know, the standard W questions. Who, what, when, where, why, and how. One feature stands out before the rest, that I see. Language. Artful facility with language's diction and syntax and rhetoric's schemes and tropes. I had a duh-huh moment epiphany when I realized the better flash fiction I read reads to a degree like poetry, albeit, without a rhyme and meter scheme: free verse, blank verse, slant verse. Though both forms are typically short forms, barring, of course, epic poetry.
A similarity of note between poetry and flash fiction: Artful excess of language. Yet not poetry due to two qualities; one, rigorous avoidance of traditional poetry's rigid principles: an aesthetic feature. Flash fiction should not read like poetry. The poetry of flash fiction should be invisible. Two, formatting in paragraph structures, not verse, line, and stanza: a mechanical feature. Flash fiction should not look like poetry.
Other aesthetical features shared between flash fiction and poetry include poetry's subtler features; like accentual verse, alliteration, syncrisis, enjambment, hyperbaton; more than mere metaphor or simile: reverberating irony, symbolism, and imagery; repetition, substitution, and amplification; and turns, caesuras, and medial pauses. Turns, that's the E-ticket to ride. Turns fall at distinct divisions within poetry and flash fiction.
Take the sonnet form, English or Italian. A major turn must take place at about the three-quarters mark through a word count, about where a final stanza begins. Yet each paragraph or stanza must have at least a minor turn as well. Turns are challenging for writers and poets.
What's a turn? A feature that expresses a sense of time, space, and transformation movement. For example, a roll of red tickets. The image that limited description evokes might be anything. How about the roll has unspooled in a car trunk and though still potentially useful is more or less ruined. That evokes a stronger image. More importantly, that expresses a sense of time, space, and transformation movement. Time and maybe space—the spool is out of place—have moved through the object and transformed it.
Where poetry uses line breaks and stanza breaks to create emphases, structure pace, and signal turns, prose uses paragraphs and punctuation, like commas, periods, question marks, colons, semi colons, and em dashes. Ellipsis points—well, as often misused as exclamation marks . . . Not that poetry doesn't also use punctuation, only prose's punctuation often is deployed more and often in place of line breaks.
And the subtlest of all features of more artful and in-demand poetry and flash fiction: implied meaning that readers can readily infer while reading a first time. H.P. Grice calls this implicature: an implied meaning that's readily understood and valid or at least credible. An example;
"Are you going to Marty's birthday party?" Dave said. "I have to work," Beth said.
Beth didn't say yes or no. She answered with a non sequitur reply. Logically, working has nothing to do with going to a party. Rationally though, Beth's response says she cannot go because she has another obligation. However, Beth leaves open whether she will, in fact, attend the party. Surprises are wonderful. Heck, she might blow off work and go. I have.
Emily Dickinson says best what implicature means and how and why it works to engage readers. Don't state meaning directly. Get there through artful misdirection.
Tell all the Truth but tell it slant— Success in Circuit lies Too bright for our infirm Delight The Truth's superb surprise As Lightning to the Children eased With explanation kind The Truth must dazzle gradually Or every man be blind—
But then again, for that matter, all the above may equally apply to longer fiction as well.