I was asked this at a convention I was honored to be a guest author. They key to a hard-hitting short story is to grab reader immersion immediately. Put the character in a relatable situation or something familiar.
For example, have them pause in front of a door. Instead of describing them pausing to knock, start dicing into emotions, internal dialogue, overhear something going on inside, or even body language to reveal something that ties into the plot.
Like @IndigoShirtProd mention, focus and double-down on a singular plot. Unlike a novel that needs subplots to build and strengthen an idea in the long game, you are at the turning point and climax of a jarring thing. Flash fiction exposes this the best!
Here's an example I wrote at a writing exercise and got instant reaction for I titled "Tradition" after the prompt:
The tradition was I follow my father’s words without question.
Once, just once, I gave him a disapproving glare and was left with a bruised cheek. My fingers tightened their grip on the hairs of the old Shire’s mane. The draft horse was clumsy yet strong as he galloped down the road. His hooves beat against the cobblestones in resonance with my heart.
‘Never again.’ I lipped, hot tears still snaking down my cheeks. ‘No more.’
I wore my purple badge with pride. Each passing moment put more distance away from a prison my family called tradition. Chills rippled across my skin, my mind recalling the green-toothed grin of a man much older than my father. On the table was a fine leather bag full of coin. I knew what was happening as I was asked to curtsy and speak my name.
“What a lovely daughter.” Chuckled the man in fine clothing. “A fine bride, indeed.”
“She knows her place.” Father’s hand ripped across my face and I held my tongue. Tradition. “Silent and strong. A deviant look in her eyes, though.”
“Impressive.” The man sounded like a snake, hissing, “Most excellent.”