Help: Name for zooming a small area of a photo or diagram and placing it within a square within the original image.

I see these a lot in science books and technical manuals. The small area is encircled and then has lines leading away to a larger circle that is enlarged. What is this process called and how can I learn to do it?

Great design resource

100 Things Every Designer Needs to Know About People (Voices That Matter) Submitted December 12, 2016 at 02:23PM by griffcon http://ift.tt/2hpeF69