A film editing technique. Like the "J-cut" or the "L-cut" it's used when the sound occurs prior to or not over the sync visual. Similarly, the definition of a K-cut is when you use sound over black before bringing up the sync image. It's called the K-cut for two reasons, the first because it's used over black and K is the letter designated to black as in CMYK. The other reason is because it's named after the editor who coined this term, Kai.
The use of K-cuts in that horror movie really made it much scarier because it left the visual to the viewers imagination.