bending is similar to a lobotomy. Amon might literally sever a specific nerve cell and
thus, sever the connection between the respective elements.
Since Amon has never taken away an airbender's bending, or an Avatar's bending, he could have made a
mistake, and only severed Korra's connection between earth, fire and water. When she tried to bend, her brain may
have been forced to use nerve pathways that it had never tried to use before.
These are my ramblings.