Amon used a physical alteration to the body that resulted in a 'disrupted' chi path using blood bending. Unlike Chi Blocking, which is takes a while to wear off and can be undone with healing, this method was permanent unless undone by a) another blood bender making the path resume its regular flow or b) it being forced open via energybendnig. This was also noted in the fact that this technique required touching the victim's forehead, which is an opening of one of the eight gates as explained by Guru Pathik, thus a direct line of contact with the chi of an individual. 

Essentially, it was the latter, but it does have limitations and implications.