Well, in these short horror stories, the element of surprise is crucial. It could be a sudden appearance of a ghostly figure or a hand out of nowhere. Also, the use of senses helps. A cold touch, a strange smell, or a terrifying sound can enhance the horror. Moreover, a sense of isolation, like being alone in a scary place, makes it scarier. It makes the reader or listener feel more vulnerable to whatever horror is coming.
The sudden and unexpected events. For example, a character thinking they are alone and then suddenly seeing a shadowy figure right behind them.
The sudden appearance of unknown entities. For example, a figure suddenly showing up in a supposedly empty room.
Once upon a time, I walked into an old, abandoned house. The floorboards creaked ominously. In the corner, a shadow moved on its own. I felt a cold hand on my shoulder, but when I turned, no one was there. Then I heard a whisper, 'Leave while you can.'
One key element is a sense of dread. For example, in a story about a cursed object, the main character starts to feel an unexplained unease as soon as they come into contact with it.
One 'Shitty 2 Sentence Horror Story' could be 'I woke up to find a handprint on my window. It was on the inside.' The horror here is the sudden realization that something or someone was inside the room, close enough to leave a handprint on the window, which is a very private and supposedly safe space. It gives a feeling of violation and immediate danger.
The suddenness is really scary. For example, in stories where a normal situation suddenly turns terrifying like a quiet room and then a loud, unexplained noise. It gives no time to prepare.
The suddenness. For example, like 'The phone rang in the dead of night. It was a voice from the grave.' The sudden phone ring in the middle of the night is already scary, and then it's a voice from the grave which is very spooky.
Another really scary one could be 'My phone rang at 3 am. The caller ID said it was me.' It plays on the idea of the self being something other or unknown, and calls at that time are always spooky.
Some might be based on real experiences or fears, but many are fictional creations for entertainment.