Who ever it is; he has not only knowledge of chi blocking but of the spiritual plane. He did not talk to spirits but probably a guru spiritualist. He learned about how to open and seal the third eye. The spiritual side of bending. Thats why he puts his thumb on the middle of their forehead, third eye. It's like how Iroh invented how to redirect ligtning and combustion man (he actually had the third eye painted on his head), by exploring all sides ofelements like what aang did with the guru. Since most of the people in republic sity only think of bending as physical; they don't realize amon is using the third eye. So Korra who has always been weak against the spiritual side of bending won't be able use bending when he seals he third eye. This also explains why she won't be able to tap the avatar state and defeat him. He is from what seen a spiritual guru that learned chi blocking. What do you think?