Coined by extension from flashback, the flashforward entails, as usual for literary devices, a mechanical aspect, an aesthetic aspect, and a synthesis of both that is profound and sublime. Charles Dickens' A Christmas Carol illustrates the flashforward for the ghost of Christmas future segment, and is much emulated since.
This is what the future does hold if Scrooge does not mend his wayward ways! The mechanical aspect is that the future of Scrooge is certain, depicted as already come to pass, but for a substantive personal maturation adjustment within the present time. The sort is an extended subjunctive grammatical mood; "of, relating to, or constituting a verb form or set of verb forms that represents a denoted act or state [of being] not as fact but as contingent or possible or viewed emotionally (as with doubt or desire)" Webster's.
The Wikipedia "Flashfoward" article contains a few clues to the form, yet, as is often the case, mostly superficial, mechanical content. The article does, though, connect the form to foreshadowing's aesthetics without detailed explanation of the connection. It is Wikipedia, after all, and the mechanical-aesthetic-synthesis relationship is subject to near infinite expression, most so a synthesis of the mechanics and aesthetics such that the synthesis transcends the two and expresses a greater truth underlay.
Silva Rhetoricae's "Prolepsis," synonym of flashforward, notes: "2. Speaking of something future as though already done or existing. A figure of anticipation." This is the true aesthetic aspect of the flashforward for prose purposes -- present-sense impression of the future as though extant. Futuristic science fiction especially attempts prescient perception and, more often than random chance, succeeds, though not limited nor exclusive to the genre. Literary analysts assert the flashforward is a Postmodern literature convention: questions and challenges of presupposed proprieties.
What about answers or satisfactions thereof? The synthesis accomplishes such. What does the near- or long-term future hold for the human condition that is drama worthy? Something present-day that is as yet unrealized! Digital technology's perilous depersonalization phenomena, digital warfare, pervasive state and nonstate privacy intrusion surveillance, evermore short and long tail Trickle-up economics, everyone's fifteen minutes of fame? Near infinite possibilities, alas.
The flashforward's synthesis or, more specifically, prolepsis that is drama worthy might, though, could even, perceive a non-imminent present-day hazard as come to pass and extant in the future. Isaac Asimov's The Naked Sun (Solaria), George Orwell's 1984 (Big Brother), Ray Bradbury's Fahrenheit 451 (Censorship, Technology destroys social culture, Majority rules' force majeure disinhibits social irresponsibility).
Nor is prolepsis limited to futureward events, alternative concurrent timelines could serve as well. Likewise, no absolute that prolepsis depict future circumstances as already extant, they need only imply those are possible and likely, such that doubt of outcome comes into question, like Scrooge's pendent larger-than-life, life-altering change of attitude. He could have refused, after all, and decided the miserable, lonely, legacy-less end was all right with him; he'd lived well enough and would not change regardless.
Pastward prolepsis is also possible, the way humans reconsider past circumstances in more or less favorable lights than those really were. Present-sense impressions of past events invariably differ from past present-sense impressions. Why not fully realize that scenario for prose purposes? Like the way H.G. Wells' The Time Machine uses prolepsis to perceive the future as like history accounted forward in time, based upon pseudo-social science psychohistory. "Those who cannot remember the past are condemned to repeat it" (George Santayana, 1905, Reason in Common Sense, The Life of Reason Vol.1). And repeat it in evermore escalated emphasis and to greater and greater harm until . . .
Synthesis for drama worthiness means that prolepsis' mechanical and aesthetic aspects be emotionally engaging and propel antagonal, causal, and tensional dramatic movement toward a bittersweet satisfaction end. No more, no less.