493bbc30__Touch_Gesture_Reference_Guide__Platform_Description
[Platform] iPhone OS [Platform Gesture] Pinch Open [Core Gesture] Spread [Platform Description] 
A two-finger user gesture used to zoom in. To pinch open, users place the thumb and a finger (or two fingers) close together on the screen and move them apart without lifting them from the screen.
LukeW | Touch Gesture Reference Guide
Platform Description