During chapter 18, Mayella takes the stand and testifies against Tom Robinson, claiming he raped her when she invited him inside to bust up a chiffarobe. Her description of the attack is initially not terribly detailed:
... I turned around an 'fore I knew it he was on me. Just run up behind me, he did. He got me round the neck, cussin' me an' sayin' dirt—I fought 'n' hollered, but he had me round the neck. He hit me agin' an' agin'...
After a brief pause, she finishes with:
he chucked me on the floor an' choked me 'n took advantage of me.
Mayella's testimony does not hold up under scrutiny. Atticus questions Mayella about her home life, from which it is revealed that Bob Ewell is abusive to his children, and he asks her to repeat that Tom choked her and hit her face before revealing that Tom has a withered arm—making the attack as described by Mayella physically impossible, not to mention her testimony quite invalid.
Mayella's conduct during the scene is agitated. She is clearly under pressure by her father, looking to him when Atticus's questions make the ruse more difficult to uphold.
Mayella ends her testimony with an outburst:
I got somethin' to say an' then I ain't gonna say no more. That nigger yonder took advantage of me an' if you fine fancy gentlemen don't wanna do nothin' about it then you're all yellow stinkin' cowards, stikin' cowards, the lot of you.
This is meant to deflect all the evidence proving she has been lying about the rape. She is using an emotional appeal to get the jury on her side: her words evoke the idea of white men needing to protect white women from the sexual advances of black men. Though she is lower class and therefore considered "white trash" by the more privileged townspeople, her race gives her an advantage over Tom since old prejudices linger in Maycomb. She is basically claiming any refusal to convict Tom is a sign of their lack of integrity and manhood. Unfortunately, though the evidence all suggests Tom's innocence, these appeals override any reason or sense of justice in the Maycomb jury.