Which rhetorical appeal uses speaker credibility to persuade an audience

Read the following excerpt from life on the mississippi by mark twain. no girl could withstand his charms. he "cut out" every boy in the village. when his boat blew up at last, it diffused a tranquil contentment among us such as we had not known for months. but when he came home the next week, alive, renowned, and appeared in church all battered up and bandaged, a shining hero, stared at and wondered over by everybody, it seemed to us that the partiality of providence for an undeserving reptile had reached a point where it was open to criticism. how does twain's use of understatement serve his purpose? be sure to use specific details from the text to support your answer.