Stacking - also known as coloring, painting, ect... - is the act of using as little space as possible in order to outlive your oponent. In order to do this, players will double tap their screen to make the smallest 180 degree turn possible.
Types of stacking
Two Finger stack:
The itouch is placed on a surface as the index and middle fingers hover over the screen. First the index then the middle finger will tap the screen. This is the most commonly used method used by Beanz, Sky , Xenon, ect...
Double Thumb stack:
The itouch is held in hands as only the thumbs are exposed to the screen. When a stack is to be made, both thumbs meet on the side they are going to hit and tap together. This method is used by Adambark.
The Reburm stack: