What is Atticus' strategy or plan of defense in the Robinson case?
In To Kill a Mockingbird, Tom Robinson is accused of raping and attacking Mayella Ewell, and Atticus is consequently appointed as Tom's defense attorney for the duration of the court proceedings.
During the trial, Atticus calls Mayella's father, Bob Ewell, up to the stand as the second witness, which some might consider to be a risky move given the fact that Bob is the town drunk and an incredibly unstable personality. However, Atticus' plan becomes clear after he asks Bob to write out his name before the court. Bob does so, which reveals an incredibly significant detail: the man is left-handed.
Due to Mayella's injuries being on the right side of her face, the court knows that a left-handed man must have perpetrated the attack. After Atticus establishes that Bob is left-handed, he proves that Tom could not have attacked Mayella by asking him to stand and reveal his left arm, which is horribly mangled and useless--the product of an accident as a young man.
Thus, Atticus' plan of defense is one of strategic elimination. He gets rid of the possibility that Tom could have committed the crime while casting suspicion on Bob and leading the jury to a new conclusion: that Bob himself attacked his own daughter.
If you are looking for this in Chapter 9, you are going to need to look at the end of the chapter where Atticus is talking to Uncle Jack. He doesn't say a whole lot about the strategy, but he says a little.
First, Atticus says that it is a case of Tom Robinson's word against the Ewells' words. He does not think he can win that way. So we see his defense is to try to make the Ewells look like liars and to use Tom's words in his own defense.
He also says he intends to shake the jury up a bit, but we do not really know what he means by that.