In the Hitchcock zoom, the setting of a zoom lens is used to adjust the field of view at the same time as the camera moves towards or away from the subject in such a way as to keep the subject the same size in the frame throughout.
Thus, during the zoom, there is a continuous perspective distortion, the most directly noticeable feature of which is that the background "changes size" relative to the subject. As the human visual system uses both size and perspective cues to judge the relative sizes of objects, seeing a perspective change without a size change is a highly unsettling effect, and the emotional impact of this effect is much greater than the description above can suggest.
The Hitchcock zoom is commonly used by film-makers to represent the sensation of vertigo, or to suggest that undergoing a realization that causes them to reassess everything they had previously believed. A notable use of this effect is in Goodfellas, where director Martin Scorsese uses the Hitchcock zoom in a scene during the climax of the film: Henry Hill (Ray Liotta) and Jimmy Conway (Robert De Niro) are sitting in a restaurant, talking. Henry realizes that Jimmy is setting him up and betraying their lifelong friendship; as this happens, the perspective in the background changes in a slow, gradual manner.