according to the instructable:
Shadow knitting, or illusion knitting, is a process of knitting such that the finished piece has a hidden image only viewable from an angle. The effect is created by alternating rows of two colored yarns so that the raised stitches from one row block out the flat stitches of another row. The finished work looks like a simple striped pattern from the front, but when viewed from an angle, the “hidden” image appears.
instructables is a great resource for how-tos & tips - watch videos, follow step by step illustrated guides, & post up your own projects/results if you want - all for free!!