Just a thought concerning everybody's favorite masked charlatan.
When Amon first reveals his signature abilities, he announces to the crowd that they came from the spirits, further elaborating that they have grown tired of the influence benders have on society, finding it necessary to grant one person the ability to remove bending.
Now, when he is revealed as a fraud (sort of), the "it came from the spirits" explaination is never really adressed: Amon is revealed to be a bender, fight ensues, he shows off his other skills to his supporters, he flees. The overwhelming concensus is likely to be that his initial explaination of his abilities was purely an invention for him to gain credibility, but lets really look at this situation for a moment. How are some ways that Amon could have aquired his unique bloodbending skill?
If it were through experimentation, chances are someone would have noticed people getting spirited away to some bloodbending lab. The human body is extremely complex, and the physiological ties to the mystical art of bending is not fully understood, so if Amon had gotten his bender-be-gone ability the old fashioned way, he would have had to use many human subjects, which most likely would have led to unwanted attention. Even if the United Forces or local law enforcement couldn't stop him, there would likely be a "mad bloodbender on the loose" across all media, along with the sorts of victims he would leave. Not exactly an ideal situation when you want to re-invent yourself.
It likely wasn't through any ancient discoveries, as bloodbending hasn't existed that long. It also probably wasn't through any sort of human mentorship, as Amon seems to be the most knowledgeable bloodbender ever known.
Now we come to divine intervention. I know its a bit of a stretch, but just bear with me on this one. The spirits have proven knowledgeable about bending in the past, shown by the Ocean Spirit and Wan Shi Tong. Even if they can't bend themselves, they still know of its techniques, and probably know more about the art than anyone on the material plane given the fact that, barring accidents, they are immortal. It has also been shown that spirits are able to energybend (Avatar spirits specifically, but since it is the only bending skill spirits can use, it is probably known to at least a few other spirits), which can impart knowledge to the user, bending or otherwise.
Now that we know the basic plot of the next book of LoK, as well as certain spirits from AtLA *coughKohcough*, we also know that certain spirits can be less than amicable when it comes to certain parts of the material plane. Koh, for instance, took personal offense when the Avatar chose to neglect his duties in peacetime. It isn't too far of a stretch, I think, that a few spirits may either share the sentiment of most Equalists in that bending gives people too much power over their fellow man, or indeed they might be jealous, as spirits cannot bend on their home turf.
Now, with that bit of speculation, it isn't a terribly large stretch to assume that Amon may have actually told the truth about his unique bending abilities, from a certain point of view. A rogue spirit, emboldened by the lack of a spiritual enforcer, could have given Amon the knowledge of how to imitate energybending, the only sort of bending spirits can do, with bloodbending, the only other bending style that can manipulate the body that is not held exclusively by the Avatar.
Thoughts? Comments? Concerns?