This is an excerpt from Fundamentals of Motor Behavior by Jeffrey Fairbrother.
Although we often use information from a variety of sensory sources, it is sometimes helpful to isolate the role of individual senses to gain a better understanding of the extent to which they guide movement skills. Because vision is usually our dominant source of sensory information, researchers have developed many ways to explore how we use vision. One of these techniques is called visual occlusion. In this technique the researcher hides body parts or movements, often via video or film editing, in order to study how people use vision to anticipate the best course of action.
Suppose you want to know how an expert tennis player uses visual information to respond to an opponent's serve. One question that might emerge is whether or not experts are simply better at tracking the tennis ball than novices. On the other hand, it seems likely that experts learn to anticipate the serve because the speed at which the ball travels simply does not allow much time to respond. To test this proposal, you could show experts and novices a video clip of a service from a receiving player's perspective, but stop the clip at the moment of ball contact. If visually based anticipation does not distinguish between the experts and novices, they should be equally successful at predicting the location of the serve. If visual information preceding ball contact is used to anticipate the serve, you would expect the experts to have more accurate predictions, which is actually the case. To get an even clearer picture of how experts use visual information, you could stop the video clip at earlier and earlier points until you find that the predictions are no more accurate than chance. As it turns out, experts are better than novices at extracting information from visual cues related to the opponent's posture during the portions of the serve preceding ball contact (Williams et al., 2002, 2004).