This is an instance where an American (usually an African-American) is just about to have sex with the same sex as the person. Before any article of clothing is taken off (during the presex makeout) one of the people takes their finger (usually thumb) and shoves it in the ass of the other person. Usually the response of the other person is, "WHHOOOOOOOAAAAAAAAAAAAAA ! ! !" (This is the usual response of Dirty Dan during American Finger-Fisting.)
When watching Naruto, I saw some American Finger-Fisting when Kakashi does Thousand Years of Pain.