Amon is an incredibly advanced bloodbender. Like many highly specialized benders he can sense his element without having to see it. Regular waterbenders can sense/control water out of their sight, earthbenders can feel vibrations through the earth, and firebenders can likely sense heat. In this picture Naotok can feel his brother’s blood, feel him reach for the gas cap. He knows what Tarrlok is about to do.
And he lets him do it.
They have both been steeped in revenge for so long that Amon thinks the only way for him to properly make it up to his brother is to let him have his vengeance.