They are effective because they create immediate shock. Just one sentence can paint a very disturbing picture in your mind. For example, 'My reflection didn't move when I did.' It makes you wonder and feel unease right away.
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.
Well, it's all about the pacing. In a '5 sentence horror story', each sentence has to move the horror forward. The first sentence sets the scene, the second adds a bit of unease, the third builds more tension, the fourth takes it to a peak, and the fifth leaves you with a sense of dread. This tight structure doesn't give your mind a chance to relax, and that's what makes them so effective at sending shivers down your spine.
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.'
One could be 'I entered the abandoned asylum. A cold hand grabbed my ankle.' Another is 'She thought she was alone. But the eyes in the mirror said otherwise.' These short horror stories create a sense of dread quickly with just two sentences.
The brevity of '2 sentance horror story' is key. It leaves a lot to the imagination. For instance, 'I opened the attic door. A strange smell and a soft whisper greeted me.' The lack of details makes our minds fill in the blanks with the scariest things we can think of. Also, they can end on a cliffhanger, making the horror linger in our minds.