Mayella Ewell is called to the stand. She testifies that she asked Tom Robinson to chop up a dresser in return for a quarter, and that when she turned around Tom attacked and raped her. In cross-examination, Atticus shows that Mayella is terribly lonely. When Atticus asks Mayella to identify Tom, and Tom stands up, it becomes clear that Tom's left arm is useless: it was destroyed in an accident. If Tom can't use his left arm, then how could he have beaten and raped Mayella? Atticus asks Mayella wasn't Bob Ewell the person who beat her? Mayella refuses to answer.
Now the evidence is definitive: with just one arm, and that arm his right arm, it is physically impossible for Tom to have battered the left side of Mayella's face. If the jury convicts Tom now, the only possible explanation is that racism either consciously or unconsciously affected the views of the jury-members.