The brevity. It gets straight to the horror without a lot of build - up. For example, 'I opened the attic door. A cold hand grabbed my wrist.' Just two sentences but it creates a sudden sense of dread.
The first one: I heard a strange noise outside my window. I peeked through the curtain and saw my own reflection with a sinister grin. Another one: I woke up in a cold sweat. There was a figure at the foot of my bed, but when I blinked, it disappeared.
They are effective because they create a sense of mystery and fear quickly. In just two sentences, they can set a spooky scene and leave the reader's imagination to run wild.
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.
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.
I think 'The clock stopped at midnight. Then I heard footsteps.' is quite scary. It gives this feeling of something ominous starting right at a spooky time.