The dolly zoom is a mesmerizing visual effect that is immediately recognizable in use. And while it's an impressive trick, it can carry with it a lot of information that can convey emotion, important plot points, or imminent danger.
The dolly zoom was first used in Hitchcock's "Vertigo" to convey Jimmy Stewart's dizzying fear of heights. The technique relies on zooming a lens in while dollying the camera backward (or vice versa). This results in the subject remaining the same size, but the angle of view changing, which causes the size of the background to change while that of the foreground remains constant. The effect is disorienting and stark, as it's impossible in natural perception. And while it's an impressive slice of visual trickery, the effect doesn't end there. Directors use the powerful impression it leaves to either prime the viewer for further emotional responses (or create one immediately), as Hitchcock did by using it in "Vertigo's" opening and climatic scenes, or as a marker of some sort of important plot point. Few shots have such an arresting quality, so when used judiciously, it can a great way to draw immediate attention to something you want the audience to notice.
[via No Film School]