Prior to discussing the injuries sustained by Miss Mayella Ewell during the alleged altercation, Atticus had Tom Robinson stand so that Mayella could "have a good long look" at him. Scout's reaction to Robinson's physical appearance presumably mirrored that of others in the courtroom who did not know Robinson; she was shocked to see that Robinson
"...looked oddly off balance, but it was not from the way he was standing. His left arm was fully twelve inches shorter than is right, and hung dead at his side. It ended in a small shriveled hand and from as far away as the balcony I could see that it was no use to him."
Atticus used this opportunity to provide the jury, as well as everyone else present, with the visual information that should have led them to understand that Tom Robinson, with only one functioning arm, would have been unable to choke Mayella Ewell. This fact should have raised sufficient reasonable doubt to have led to Robinson's acquittal, especially considering Bob Ewell's testimony stating that fingermarks were found "all around" Mayella's throat. Robinson's disability clearly revealed his innocence.
In Chapter 17, Atticus spends a considerable amount of time questioning Heck Tate about the location of Mayella Ewell's injuries. Sheriff Tate testifies that Mayella was beaten badly around her head and had bruises that encircled her throat. Initially, Heck Tate testifies that the left side of Mayella's face was badly beaten, but quickly retracts his statement and says that it was the right side of her face that was significantly bruised. Atticus makes Sheriff Tate repeat his statement that Mayella's right side of her face was beaten and continues to ask Tate about Mayella's other injuries. Sheriff Tate then proceeds to describe the bruises encircling her neck. Atticus wants the jury to understand the extent and location of Mayella's injuries before he reveals that Tom Robinson is handicapped. After Atticus reveals that Bob Ewell is left-handed, Tom Robinson is asked to take the stand, and the jury realizes that Tom's left hand is utterly useless. The location of Mayella's injuries to the right side of her face suggests that a man who is left-handed inflicted the injuries. Tom Robinson could not have struck Mayella with his left hand, let alone strangled her. Atticus' attention to Mayella's injuries reveal Tom's innocence.