“There’s a bit of theory on how Amon’s hand placements are different from Aang’s.
The point on the forehead both Amon and Aang touches is the location of the Light Chakra, which deals with Insight and Illusion. Aang also touches the location of the Air Chakra, the heart, which deals with Love and Grief. Amon didn’t touch Tahno’s back of the neck to remove his bending, so that doesn’t seem necessary to do. Also, in acupuncture, that point is literally called amon.
So this theory says that Amon’s energybending is incomplete, broken, and all those flashbacks Korra had were not about Yakone/Tarrlok, but to show the real and correct way of energybending. Amon doesn’t touch the heart chakra because he can’t let go of the grief of his parents’ death. Instead, he touches the back of the neck. The throat is the location of the Sound Chakra, and it deals with Truth and Lies.
(More to the realm of speculation) That means Amon isn’t hiding anything behind his mask, what he said was his motives are absolutely true. He’s not a Scooby Doo villain whose mask just had to be removed. Just as Oscar Wilde puts it, “Man is least himself when he talks in his own person. Give him a mask, and he will tell you the truth.”“