493bbc30__Touch_Gesture_Reference_Guide__Platform_Description
[Platform] GestureWorks (Flash) [Platform Gesture] Multi-point Zoom [Core Gesture] Spread (Multi-finger) [Platform Description] 
Start with both hands together and move them apart across an area on the surface to zoom in. Pull both hands closer to each other to zoom out.
LukeW | Touch Gesture Reference Guide
Platform Description