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.'
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.'
The ending can create shock if it involves a sudden and intense reveal. After the two seconds of hiding, if there is a quick and extreme display of the horror element, such as a burst of blood or a very grotesque creature suddenly showing up in full view. This suddenness catches the reader or viewer off - guard and creates shock.
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.
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 often involve being trapped, like in the riddle of the room with no exit. Trapped situations are inherently scary. It makes you feel the helplessness of the character in the riddle.