What dramatic fact do we learn about Tom Robinson at the end of Mayella's testimony?

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,

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"Then why didn't the other children hear you? 
Where were they? At the dump?
(pg187)

Mayella doesn't
answer him.

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"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.

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"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

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"....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)