Keyholing is a specific visual technique, sometimes referred to as Spotlighting,
which uses the Value Pattern "Small Dark on a Large Light on a
Midtone Background". A common Keyholing usage is the sheriff character
enterring through the doorway into the saloon, brightly backlit so that
he is in dark silhouette, his shadow elongated on the ground and the
bar itself becoming the midtones. Think back to any of the alien abduction
scenes you have viewed, the bright overwhelming light, some indistinct
form coming out of the light, getting darker with a harder edge as it
gets nearer, shadows on the floor reaching out towards the viewer...
same technique.
