Something many ATLA/LOK viewers may not realize is that opening chakras directly correlates to unlocking bending sub-skills. Allow me to explain…
In ATLA, Aang meets a man named Guru Pathik. Pathik teaches him about the seven different chakras and explains how to ‘open’ them, so that Aang may once again enter the Avatar State. Characters who unlocked a sub-skill have unlocked a corresponding chakra in the same moment. Having a chakra blocked also explains why some characters cannot learn certain skills.
1. Earth Chakra – deals with survival, blocked by fear
Lavabending. Bolin and his friends were in imminent danger; he faced his fear, confronted a wall of lava, and bent it back. His desire to live and protect others, and overcoming his fear of death, opened his Earth Chakra. By unlocking the chakra, he unlocked lavabending.
2. Water Chakra – deals with pleasure, blocked by guilt
Bloodbending. Katara was forced to bloodbend against another bloodbender, Hama, or her friends would be killed. Despite her reservations, the overwhelming need to save her friends surpassed any guilt she may have felt in that moment. She unlocked this chakra as well as this sub-skill.
3. Fire Chakra – deals with willpower, blocked by shame
Lightningbending. With Uncle Iroh’s guidance, Zuko tries to bend lightning, but he cannot due to anger and frustration. This is a byproduct of the shame Zuko carries with him. He is unable to lightningbend in ATLA because he could not forgive himself.
4. Air Chakra – deals with love, blocked by grief
Details unknown. (It is possible that spiritual projection relates to this chakra, as Jinora is the only person currently capable of this airbending sub-skill. Jinora is full of love and virtually empty of grief compared to other characters. However, since we do not know how she actually unlocked the ability, this is not confirmed.)
5. Sound Chakra – deals with truth, blocked by lies
6. Light Chakra – deals with insight, blocked by illusion
Metalbending. Toph saw past the illusion that refined metal no longer contained earth. She knew that during the forging process, metal would still contain earthen impurities. This realization allowed her to bend her way out of a metal cage.
Something to note about older Toph and the Swamp: Guru Pathik says, “The greatest illusion of this world is the illusion of separation.” A swampbender tells Aang the swamp is “One big, living organism. Just like the entire world … We all have the same roots and we are all branches of the same tree.” Toph’s ability to see past illusion also allows her to 'see’ the entire world through the swamp.
Bolin was never able to learn metalbending because he is perpetually naïve; he sees what he wants to see, not necessarily the truth. This may hold true for Aang, as well, since he couldn’t metalbend either. Korra was able to metalbend immediately, as this chakra (and the harsh reality of the world) was not blocked for her.
Also - Combustionbending. The third-eye tattoos that combustionbenders have on their forehead is located at the Light Chakra point. Combustionbending is possible by focusing a massive amount of chi into this point, then releasing it forward. We can’t confirm that combustionbending is unlocked through this chakra, only executed.
7. Thought Chakra – deals with pure cosmic energy, blocked by earthly attachments
Flight. Perhaps the most obvious unlocking of a chakra in the show. Zaheer had no attachments in the world except for his lover, P’Li. Upon her death, he immediately unlocked the Thought Chakra and gained the ability of flight.
The only sub-skills not mentioned above are seismic sense/truth-seeing, and healing, as we’ve never seen someone unlock these skills along with a chakra. Katara accidentally healed herself at random, which means she may have opened a chakra sometime beforehand. Spiritbending, as performed by Unalaq and Korra, is just another form of healing.
I hope this helps explain how some characters were able to unlock bending sub-skills seemingly randomly. *cough* Bolin.