It works just like the human eyes: due to the space between your eyes, each eye sees a slightly different image of the world. The brain constructs the vision from both eyes into a single image in the brain, using the offset of each image to judge focal distance to the subject.
Focus Sniper replaces your human eyes with side video cameras that flank the main camera lens. The video stream of each side camera is shown on a side tablet, held by an operator. The side tablet contains a video screen, with a simple interface where the operator can move a focus line back and forth across the field of vision by turning the wheel on the side tablet. The positioning signal of both side tablets are fed into the central unit.
Here's a video of it in action: