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.'
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.
In some hide two second horror stories, the ending could be a realization that the threat was much closer than expected. After the two seconds of hiding, the protagonist discovers that the source of horror, like a menacing entity, was right inside the hiding place with them all along. They only notice when it's too late as it reveals itself in a terrifying way.
By using a sudden shock. For example, a quick and unexpected death of a main character at the end can jolt the reader. Like 'She was running away from the monster, and then, boom, she fell into a bottomless pit.'
They are concise. For example, 'The phone rang in the dead of night. It was my voice on the other end asking for help.' In just two sentences, it creates a sense of dread. They often rely on the unexpected. Like 'I found a door in my basement that wasn't there before. When I opened it, a cold hand grabbed mine.' It takes a normal situation and twists it into something terrifying.
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.