It's the element of surprise. A 'two second horror story' doesn't give you time to prepare. It throws a scary idea or image at you in a flash. For example, if you quickly see a ghost - like figure for two seconds, it plays on your primal fears. There's no build - up, just immediate horror. It also often leaves a lot to the imagination. Since it's so short, your mind has to fill in the blanks, which can be even scarier. And the brevity makes it more impactful as it's a concentrated dose of fear.
Sure. Here's one: You wake up in the middle of the night. For a split second, you see a figure at the foot of your bed. Then it vanishes. Another could be: You're alone in the house. The power goes out. In two seconds, you hear a strange scratching noise at the door.
One example could be 'I opened the old, creaking door. A cold hand grabbed my wrist. Then it was gone.' Another might be 'Walking in the dark forest, I heard a growl. I turned. Red eyes stared for a split second, then vanished.'
They're effective because they quickly hit on our primal fears. In just two seconds, they can make us imagine the worst, like the unknown danger in 'I heard a noise under my bed, then silence'. It's that sudden jolt of fear.
One possible ending could be a sudden loud noise that startles the reader. For example, 'The door creaked open slowly, and just as I peered into the darkness, a deafening screech filled the air.'
One horror story could be about a haunted mirror. In the '7 second riddles horror story', there might be a mirror that shows a different, terrifying face every time someone looks into it for 7 seconds. It drives people mad as they keep seeing these grotesque images and can't escape the mirror's curse.