On what dramatic note does Atticus end his questioning of Mayella? What is her response?

Atticus' last question asks her to explain how Tom's useless left arm could hurt her on the right side of her face. Instead of giving a straight answer, she bursts into tears, tells the people not to be cowardly and be sure to convict Tom Robinson, and will not agree with Atticus that, in fact, her father had beaten her.

Which one of Tom’s hands is totally useless?