One: 'The mirror showed my reflection, but when I blinked, it didn't.' Another: 'I heard a knock at the door. When I opened it, no one was there, but the doorknob was cold as ice.'
The brevity. With just two sentences, they create a sense of mystery and unease quickly. For example, 'I woke up with a hand on my shoulder. When I looked, there was no one there.' It gives just enough to make your imagination run wild.
One 'two sentance horror story' could be 'I always sleep with a nightlight. Last night, I woke up to find it was still on, but I was in complete darkness.' It's terrifying because the contradiction between the nightlight being on yet in complete darkness creates a sense of the unknown and a break in the expected reality.
I sat in the dark room. Then, a shadow started to grow larger and larger until it engulfed me. The air grew colder and I could feel a presence, but I couldn't see anything. It was as if death itself was in the room with me, waiting for the right moment to strike. I tried to move but my body was frozen in fear.
A man was walking alone in the forest. The moon was full but the trees were too thick to let much light in. He heard rustling behind him. He started to run. But then he realized the rustling was all around him, closing in.