Complex sentence: A reverse pinch (sometimes also called "unpinch") refers to spreading two fingers (or thumb and finger) apart, and is used to enlarge a picture or zoom in on an image.
Split into simpler sentences: A reverse pinch (sometimes also called "unpinch"). The motion involves spreading two fingers (or thumb and finger) apart to signal the device to perform an action such as enlarging a picture or zooming in on an image.