Energybending



In "Sozin's Comet, Part 4: Avatar Aang", Aang uses Spiritbending to strip Phoenix King Ozai of his Firebending.

History
After discovering that the mysterious island on which he consulted his past lives was in fact a Lion Turtle, he consulted the ancient creature on what he should do about Ozai. He asks for wisdom on how to stop the Fire Lord without taking his life.

The Lion-Turtle answers, "The true mind can weather all lies and illusions without being lost. The true heart can touch the poison of hatred without being harmed. From beginningless time, darkness thrives in the void, but always yields to purifying light". The Lion-Turtle brings up a paw, his two claws touching Aang's chest and forehead, causing a green light to illuminate.

Aang doesn’t understand at first, but is comforted by this wisdom. During the battle with Ozai, Aang is in the Avatar State, and with the combined power of the past Avatars and all the elements, the Avatar is ready to deal the final blow. At the last second, Aang exits from the Avatar State decides to not end the Fire Lord's life. Ozai thinks Aang is weak and attempts to attack Aang, who defends himself and pins Ozai down again. In this moment, he realizes the ancient wisdom of the Lion Turtle, and he decides to try it.

The Lion Turtle told him that in the era before the Avatar existed, the lion turtles did not bend the elements, but life energy itself. In order to bend another's life energy, their own spirit must be unbendable, or they would be corrupted and destroyed.

It is Aang's "true heart" and "true mind", tested in the heat of battle at the moment of striking down Ozai, that allowed him to harness the wisdom of the Lion Turtle and successfully Spiritbent Ozai.

When Aang was Spiritbending Ozai, his entire body was shining blue, while Ozai's body shined orange. At first Ozai's orange spirit enveloped Aang almost completely, but at the last moment, Aang's spirit took over. Eventually Ozai's body was enveloped by Aang's shining blue spirit, and Ozai lost his bending skill. It is unlikely that past Avatars could do this, as this ability originated before the Avatar even existed, and until that moment, Spiritbending had never been mentioned or hinted at.

Dangers of Spiritbending
Spiritbending is an extremely powerful but also an extremely dangerous technique. If the bender's spirit is even slightly tainted then they will be infected by the spirit of the one they are bending. As a result the bender can be corrupted and possibly destroyed.

Trivia

 * The chanting that can be heard while the Lion Turtle speaks to Aang explaining how to Spiritbend (also when the Lion Turtle first called Aang to sleepwalk/swim to him) is a Buddhist chant "Na Mo A Mi Tuo Fo" (spellings vary).