WARNING: There are going to be SPOILERS in this post, if you haven’t seen the finale.
I have a hypothesis about how Amon was able to take away bending. I think he used blood-bending to block your chi. The chi-blockers hit your pressure points to temporarily block your chi, which makes it so you can’t bend. Amon probably adapted this technique, but uses blood-bending to make it more permanent. You can use blood-bending to break an arm, so you can probably “break” those pressure points, I guess. (This still doesn’t really explain how Katara couldn’t figure out how to reverse it, but I guess she wasn’t practiced enough with blood-bending.)
What Amon does is different from Aang because I think Aang is actually bending your chi. Chi is basically spiritual energy, so it jibes with what the turtle island was saying about this technique. Summarized: Aang bends chi; Amon blocks chi.
I think this also explains why Korra was able to air-bend. Amon blocked her chi for all fire-bending, water-bending, and earth-bending. But he didn’t block it for air-bending. So, when Korra tried to bend, all her chi was directed into air-bending. It’s as if water was pouring out of several spigots and then forced into just one.
I’m guessing Amon never really knew how to block airbending and may have failed with Tenzen too.
None of this explained how she got her spiritual powers so fast at the end. In general, I’ve been pleased with the pacing (others will disagree) of the show, but I thought everything resolved itself way too quickly at the end. (Tarlock loses his powers way too fast, and then it’s a whirlwind from there.) In fact, I’m disappointed the season didn’t end on a cliffhanger.
I was definitely disappointed in the finale as well. My two issues with Amon using blood bending to take away bending is this: if he disrupted/blocked the flow of blood permanently, how would that not kill someone? Also, how can the effects still be acting upon someone when he isn’t present? It doesn’t make much sense. Even if what you said, about “breaking” pressure points, covered it— how do you “break” a pressure point? And if the link between blood and chi is as synonymous as you’re positing, that should have been explained, because I would think that blood and chi were two entirely separate things. Also, the fact that Amon would know how to block the other elements but not air doesn’t make much sense either. How different could blocking air-bending have possibly been? So many shenanigans.
*analogous, not synonymous