Overshadowing is a technique developed at the Australian Equine Behaviour Centre to habituate horses to objects and situations they find scary. Habituation is a natural form of learning, in which an animal, after a period of exposure to a stimulus, stops responding.
The overshadowing habituation technique is based on the scientific principle that when the horse is receiving two or more stimuli at any one time, one of them will overshadow or override all others. Overshadowing applies to people as well! As much as we like to think we can do two things at once, the truth is we can only concentrate on one at a time.