It's Asami's idea.
"In the Northern Water Tribe," she tells Korra, "it used to be that women weren't allowed to use bending for combat, and men didn't know how to use bending to heal."
Korra knew this from listening to Master Katara. "Yeah, and?"
"And that problem wasn't solved by taking away the men's power to bend in combat, or the women's power to bend to heal," Asami says patiently. "It was solved by giving the women the chance to fight with bending. By teaching the men how to heal."
Korra's a jock, not a scholar like Asami. "Where are you going with this?"
"Won't that just burn Amon's balls," Korra says.
Korra throws herself into Tenzin's lessons. The first time she airbends is an utter surprise to everyone—it's in the middle of an exhibition match against the Rabbiroos, and Korra gets knocked back a zone because she's only allowed to waterbend, and the Fire Ferrets go on to lose the match because Korra's so distracted, but meanwhile the Rabbiroos' firebender went from Ferret territory to falling into the pool behind Rabbiroo territory because Korra sneezed.
It's kind of awesome.
That obstacle conquered, Korra meditates like several thousand lives depend on it. (Mako and Bolin have nothing if their bending is lost. Not pro bending, not Mako's job at the power plant, not food money or a roof over their heads, nothing.)
Avatar Aang does not like Asami's idea. Not the least little bit. Korra proposes executing all the Equalists instead, and Aang flinches and starts telling Korra everything she needs to know.
Korra might actually have learned something from Tarrlok.
"You need someone to test this on, right?" Asami asks Korra.
"Duh," Korra says.
"How about me?" says Asami.
Korra's jaw drops. When she recovers, she says, "But I don't know if it'll work—what if something goes wrong?"
Asami shrugs. "Better me than someone who doesn't know what they're getting into. Definitely better me than someone who can't afford the best healers in the city."
Korra nods. "How about now?"
She and Asami take a little joyride to one of the Sato houses, somewhere with plenty of surrounding land. In a clearing in the trees, Korra folds into lotus pose, and Asami awkwardly imitates her. Korra reaches inside herself and slides sideways into the Avatar State; it's always there, now that she knows what to look for.
Korra reaches out and touches Asami at the crown chakra and the heart chakra. She absorbs Asami's energy, getting a feel for the light purple color and wisteria scent of her, and then pull here, twist there, redirect the flow of energy in Asami's body to fit the pattern set by Korra's.
Korra slips out of the Avatar State, her task finished. If it has been. "How do you feel?" she asks.
"I..." Asami pauses, taking stock. "I don't know?" she says. "Different. But I don't know how."
"Let's see if it worked," Korra says, and gets to her feet. "This is the simplest waterbending kata—"
Asami's not a bender, of course. That's the point. Neither of them is surprised when Asami can't make the contents of her water bottle so much as ripple.
Korra takes Asami through an earthbending kata, then a firebending kata. Nothing. The breeze is picking up, though, and Korra tries to airbend it calm so that when she tests Asami's airbending she'll know the difference between natural wind and bended wind.
The air fights her. The way it does when she's sparring with Tenzin.
"I think we're getting somewhere," Korra says. "Try this—"
Asami copies Korra's stance and gestures, and wind whips out from Asami's palm.
One doesn't solve a power imbalance by taking power away from those who have it. One solves it by giving power to those without.