(last updated: 12/28/17)
There are many seperate forms and expressions of the natural energy flowing throughout the world, and Aura as a concept is the physical disposition of a person towards being able to control and manage the great forces of energy that flow through them and that they encounter. While Chi as a score roughly represents a character's ability to create, control and output energy into the world, Aura represents their ability to receive, direct and dissipate such energies.
Most commonly, a fighter doesn't really have to do anything specific to use aura--it is only very rarely construed as an active or maintained defense. Simply speaking, a character who is skilled with their aura will tend to be able to modify their bodily energy's flow to partially counter, or dissipate harmful chi around them. This is much like having a strong constitution; just as a person with great physical resilience has it regardless of circumstances, a character with strong energy resilience typically always has it, whether unconscious, distracted or otherwise.
Though generally a character with aura will not have any noticeable physical attributes that identify them as such, a character with a comparitively large aura score might be more 'at one' with the world around them, and may very well be particularly sensitive to the flow of chi around them.
The most powerful expressions of aura are primarily movement feats. While exceptional chi users can create tornados or wildfires, the aura user has focused their abilities within. An aura user may be able to perform feats of movement or protection deemed physically impossible; walking on water, attaching to walls and deflecting or regenerating damage to your body are all traits indicative of a high aura competency.
At its strongest, aura can create visible fields of energy around your body, blocking chi attacks or even causing parts of your own body to disappear from view (by redirecting light..) It can even modify time/space to a limited degree, allowing users to bend space around their body, subtly modifying the way physics works around them, or even modifying their perception of time to better compensate for the flows of energy around them. Users of suitable legend can even extend their aura beyond themselves, creating impediments to the flow of chi in discrete regions: Many of the mystic seals maintained by the guards, shepherds and stewards of the world are examples of extreme aura control exercised. Sometimes these custodians are merely using their own formidable aura to maintain seals left behind by fallen creators much stronger than even they!
Typically, a character with a high aura competency but a lower chi competency is unable to shape chi outside of their body, and a character with low aura competency but high chi competency represents an unrefined user of chi who is either unwilling or physically unable to grasp some of the finer aspects of energy manipulation. Practically speaking, high aura competencies often come with strong chi ability simply because one trait naturally begets the other. Many of the feats told of in stories about legends long past required both chi and aura to accomplish.