In Avatar: The Last Airbender, supernaturally gifted warriors bend, or magically manipulate, the four classical elements -- earth, water, fire and air. Benders can perform stunning feats with their elements, from blasting an opponent with flames to washing whole groups of attackers away with powerful waves. For the most part, the average bender has plenty to contend with just in learning to master the more obvious and basic applications of their elements. There are many levels of skill and experience within this normal sphere of bending.
However, some preternaturally talented benders are also able to execute impressive sub-bending skills. Over the course of the Avatar and The Legend of Korra series, audiences are introduced to notable sub-benders in each of the four nations -- except for that of the Air Nomads. Does this mean that airbending has no sub-bending skill? Or could the answer perhaps be a less obvious one, which perceptive viewers may be able to glean from Legend of Korra's storyline?
Sub-bending is most productively defined as any type of bending that falls outside the basic category of its element. The first type of sub-bending the story introduces is the healing art of waterbenders -- a relatively common skill among the waterbending population. This demonstrates the deep connection of water and the human body, which leads to the second and much rarer sub-bending skill of bloodbending. This ethically fraught technique allows the practitioner to control their opponents' bodies by literally bending the water in their blood.
Then there's earthbending, which traditionally deals with the manipulation of rocks and soil. Particularly talented earthbenders, namely the extraordinary Toph Beifong, are able to expand their mastery of the element to include related matter like metal and lava. Toph is also a practitioner of seismic sense -- a skill she learned from badgermoles that allows her to "see," despite her blindness, by perceiving vibrations in the earth.
Finally, especially powerful firebenders are able to summon an extra-hot blue flame. Azula is the only character shown to be capable of this. She can also produce deadly lightning, which gives rise to the answering skill of lightning redirection, taught to Zuko by Uncle Iroh. In Legend of Korra, two benders are introduced who can perform combustionbending, a volatile technique that allows the user to concentrate their energy through their third eye, producing an instant targeted explosion.
Of all the bending disciplines, airbending is the only one without flashy, advanced sub-bending techniques as detailed above. However, some viewers have theorized that airbenders are capable of a more subtle and profound type of sub-bending: that of spiritual connection and inner balance. It's certainly interesting that of all the four nations, only the Air Nomads have a population consisting entirely of fully-fledged benders. The most likely explanation for this is their culture's total prioritization of spiritual growth. No other society in the Avatar world is as deeply spiritual and monastic as that of the Air Nomads.
Spiritual work is an ongoing problem for Korra in her journey to realizing her potential as the Avatar. Although she easily masters fire, water and earth, everything about airbending continually frustrates and evades her. Even after she starts to get the physical forms and techniques down, it isn't until she grows spiritually that she fully comes into her own and is able to control the Avatar State.
There are a few other possible answers to this question. Free flight could be considered a sub-skill of airbending, and an exceedingly rare one at that. In Legend of Korra, it's revealed that an ancient master airbender named Guru Laghima was the first person to achieve this technique. Long contemplation of his writings allowed the villain Zaheer to master this skill as well. As far as canon has shown, these two characters are the only people ever to be able to freely fly without the aid of a glider. However, it's difficult to pin this down as a sub-bending skill, as it still technically falls under the manipulation of air.
A more likely possibility, also presented in Legend of Korra, is spiritual projection. This is even rarer than free flight -- only Jinora, Aang's daughter, is shown to be capable of it. Using this art, Jinora can spiritually travel outside her body to any location in the physical or spirit world, and she's fully able to communicate with and show herself to others. This comes in handy many times throughout the show, allowing Jinora to find imprisoned allies and discover crucial information. In a sense, this actually further proves the theory that spiritual connection is the true sub-bending aspect of airbending. Whatever the case, Jinora can and should be considered a visionary master of the art in the same way Toph is celebrated for revolutionizing earthbending.
