This principles involves the animator being fully aware of an animations weight and form in a three dimensional space. Lighting and shadows must also be taken into account as well as pose.
This post above demonstrates the principle using several images. It’s basically telling us that symmetry in animation can give a 2D illusion rather than the desired 3D effect. This can be resolves by turning a character 45 degrees from the straight-on front or back pose they otherwise might have been animated in. Or, if a face-on pose is required, rather than having each arm in perfect symmetry, having one or the other doing something else can help it look more 3D. In the post above, it describes the first image of Homer below as ‘Twinning’.