The element of surprise. For example, 'I sat on the park bench. The empty bench beside me whispered my name.' Just a simple, unexpected twist in the second line can be very effective.
These stories are scary as they create a contrast. For example, 'I lay down to be tucked in. A cold hand reached from under the covers.' The normal act of getting tucked in is juxtaposed with a very abnormal and frightening event. Our minds are not expecting something bad in that situation, so it catches us off - guard and scares us more easily.
Well, one possible 'scary two - line story' could be 'I heard a knock on the door at midnight. When I opened it, no one was there but I felt a cold hand on my shoulder.' Another could be 'The old house was empty. As I walked in, I saw a shadow that moved on its own.' And a third one might be 'She walked through the graveyard. Suddenly, a whisper called her name from an unmarked grave.'
They are effective by creating immediate tension. For example, 'The phone rang at midnight. A raspy voice said 'I'm coming'.",
"Answer2":": 'Scary two line stories' work well because they rely on the reader's imagination. In a story like 'The moon was full. A howl that wasn't from a dog echoed.', the reader has to fill in the blanks about what might be making that howl. It's this sense of mystery that makes them effective.
Walking in the forest, I felt watched. I looked up, and a pair of red eyes stared from the trees. It was no animal. There was an intelligence in those eyes that sent a shiver down my spine. I tried to run, but my feet felt like they were stuck in the mud. As I struggled, I could hear the thing getting closer, branches snapping under its weight. I knew that whatever it was, it was coming for me, and there was little I could do to escape.