Sun Tzu postulated two forces–the Zheng element, which fixes the enemy in place, and the Qi element, which flanks or encircles the enemy, either actually or psychologically. The Zheng (ordinary) force is direct and more obvious, while the Qi (extraordinary) is indirect, unexpected, distracting, or unorthodox. Using both elements ensures that decisive blows will fall where the enemy does not anticipate them, and is least prepared.
The two factors are fluid. History shows many examples when what is Zheng becomes Qi, and Qi becomes Zheng.