Aura shifting consists of shaping one's aura. It's mainly done by instinct, but it can be shaped by will. It often happens when the shifter finds themself in a dangerous environment. They'll shift, looking very much the same but their soul has changed shape, for added protection. Much like how auras have different colors, it can be shaped into an anthropomorphic shape as well.
The aura (if you can see them) takes on the shape of the animal/wereside. I've also heard that with the AS, the person can take on some characteristics of that animal, mentally, but not a total mental shift.
See also