Although photos in low key photography are very dark, it is actually about light. Or at least how the light falls.
When a subject is illuminated by a ray of light against a dark background, it is possible to take a low key photo. Here, it is important to underexpose and not rely on your camera's automatic exposure. Setting spot metering often gives the best results too.
The situation where the light source (sun) is almost at the same height or slightly above the subject is also ideal for a low key photo. In such a case, the subject gets a nicely lit edge.
While a high key image (with a predominantly white background) is perceived as soft, in a low key photo it is just the opposite. This gives the viewer a dramatic, dark and exciting feeling.
Click on a picture to enlarge.