Huge spoilers for Legend of Korra.
In season one, the Legend of Korra is based on a bad guy who is a water bender and a Blood-bender who wants to take everyone's bending and make everyone equal.
When Tarrlok kidnaps Korra and locks her in a metal crate, Tarrlok goes back to the city claiming that it was Amon's men who took her when it was actually a set up to leave Republic City. He get busted when he was witnessed blood-bending her.
When he goes back to where Korra is being held he runs into Amon and tries to Bloodbend Amon, however Amon isn't affected by Tarrlok's blood-bending at all.
Why is Amon able fight it and take Tarrlok's bending. How is that possible? Why couldn't Tarrlok blood-bend Amon?