In the book To Kill a Mockingbird, what simile does the narrator use to describe Mayella after she gives her testimony, and what is the effect?

When Mayella Ewell gives her testimony in court, Scout, the narrator, notes the young woman's confidence.  Scout compares Mayella's level of confidence to that of her father, Bob Ewell.  His confidence had been brash.  Mayella's confidence is "like a steady-eyed cat with a twitchy tail" (To Kill a Mockingbird, Chapter 18).  The simile shows Mayella's wavering confidence as she gives her testimony.

Mayella states that she feels verbally attacked when Atticus questions her.  She becomes very upset.  This shows that she is not completely confident in her own story.  Looking at Mayella with the view that she is making up aspects of her story or repeating what her father told her to say, one can understand why her confidence is a wavering one.  Mayella feels the need to protect her father, as well as to stay consistent with her testimony.  Atticus asks her direct questions, which effects her confidence.