Some things I keep coming back to about flash fiction, after reading more of it than is probably healthy:
The best flash fiction examples I've read don't feel short. They feel complete. There's a difference, and it took me a long time to understand what that difference actually was. A short piece can feel like a fragment. A complete one just stops at the right moment.
Augusto Monterroso wrote what's often called the shortest story in Spanish literature: "Cuando despertó, el dinosaurio todavía estaba allí." When he awoke, the dinosaur was still there. Seven words. Entire libraries of interpretation have been built around those seven words, and none of them agree with each other, which might be the whole point.
Readers of micro fiction are a specific breed. They like re-reading. They like turning a sentence over in their mind for hours. Most people who say they love reading actually love consuming story. Flash fiction readers love the sentence itself.
A lot of writing advice tells you to cut the first paragraph. In flash fiction, you often cut the first paragraph, the last paragraph, and everything in between that isn't load-bearing.
Carmen Maria Machado's work in Her Body and Other Parties taught me something I hadn't expected: flash-length prose can carry horror better than a 300-page novel, because there's no room for the reader to catch their breath, no subplot to hide behind, just the dread and nowhere to look away.
I'm still not sure whether flash fiction is its own genre or a compression technique you can apply to any genre. I've seen people get genuinely angry about this question at literary panels. I don't think there's an answer yet, and maybe that's fine.
The worst flash fiction reads like a longer story with the middle ripped out.
Dino Buzzati, the Italian writer, could compress an entire existential crisis into a single page. His parables feel like bad dreams you can't quite shake. He was also a painter and a journalist, and I think you can feel both of those other lives in the way he built his sentences, every word doing double or triple duty.
If your flash piece needs a twist ending, it probably hasn't earned its length. The stories that stay with me don't twist. They just land.