We learn that Mayella, even though she said she screamed the whole time she was supposedly being attacked by Tom Robinson, did not scream until she saw her father's face in the window. We also learn that Bob Ewell beat her up, not Tom Robinson. When Mayella tells the court that she was screaming the whole time, Atticus asks her,
"Then why didn't the other children hear you? Where were they? At the dump? (pg187)
Mayella doesn't answer him.
"Why didn't your screams make them come running? The dump's closer than the woods, isn't it? "(pg 187)
Mayella still doesn't answer him. So Atticus pursues another question.
"Or didn't you scream until you saw your father in the window? You didn't think to scream until then, did you? (pg 187)
During this whole series of questions, Mayella keeps quiet. Finally Atticus asks her,
"What did your father see in the window, the crime or rape or the best defense to it? Why don't you tell the truth, child. Didn't Bob Ewell beat you up?" (pg 187)
By "the best defense to it" Atticus is saying that Mayella did not start screaming until she saw her father's face, so that she looked innocent. She tried to hide the fact from her father that she initiated the contact with Tom. However, her father knew, and he was the one who beat her.
This section of questioning aroused Mayella so much that she made her last famous speech to the group and then she refused to answer any more questions. She told them
"....The nigger yonder took advantage of me an' if you fine fancy gentlemen don't want to do anything about it, then you're all yellow stinkin' cowards, stinkin cowards, the lot of you." (pg 187)
After Mayella's testimony, Tom Robinson takes the stand, and during his cross-examination he says something that makes everyone in Maycomb nervous: that the reason he stopped to help Mayella break up the chiffarobe was because he "felt sorry for her". In 1933 Alabama, or anywhere in the South, for that matter, the idea of a black person feeling sorry for a white person was absolutely unheard of; it contradicted everything everyone took as fact, namely that inherent superiority of whites over blacks. It is safe to say that Tom Robinson's admission that he felt sorry for Mayella did not win him sympathetic points from the jury, but rather probably irritated some people as further indication of the "uppity Negroes" getting too confident in the neighborhood.