The principle of dolly & zoom comes from cinematography, where this effect is used to create unusual visual perception. It's achieved by doing 2 things at the same time:

• Dollying or changing the distance between the camera and subject. Think of a photographer approaching an element to take a more detailed shot.
• Zooming or scaling an image artificially. Think of a photographer using the camera’s zoom feature to enlarge certain elements.

Designers often use this principle to create a close-up on an image or change the perspective and show the image from another angle.