Lights Out - one of the scariest horror short films!

This is a great example of horror filmmaking - in less than 3 minutes.  Using just two rooms, two lights and one actress, Swedish director David F. Sandberg is the talk of the town among hungry agents and managers.  Why do you think this short film is so effective?

Lights Out - Who's There Film Challenge (2013) from David F. Sandberg on Vimeo.