During his cross examination of Mayella, Atticus's strategy is to extract a confession. What does he want her to admit?

When Atticus begins his cross examination of Mayella, his
usual courtesy and respect is evident, but it becomes apparent that he does have a
strategy.  As he zones in on the actual incident involving Tom Robinson, he establishes
that Tom Robinson has a bad left arm that couldn't possibly have been used to create
Mayella's right side black eye, that Bob Ewell is constantly drunk, and then asks if in
fact Ewell beats on Mayella.  Although Mayella doesn't outright admit it, she doesn't
refute the idea that her father beats her in any sort of believeable manner, but instead
screams that the jury would be crazy not to convict Tom Robinson.  After she loses her
composure, she refuses to answer further questions.