What does Atticus want the jury to note about Mayella’s bruises?

Atticus wanted the jury to note the location of Mayella's bruises, which Sheriff Tate says were on "her right eye...that side of her face." He goes on to say that "her arms were bruised" and that she had "finger marks" around her throat (170). The point of this becomes clear as he shoes that Bob Ewell is lefthanded, and that Tom Robinson lost the use of his left arm when he was young. Atticus has him stand in the middle of Mayella's testimony to demonstrate that his arm simply hangs beside his body. It seems very unlikely that he could have been the one that left the bruises around Mayella's neck, and that Tom could have punched her on the right side of her face, since his left arm (obviously it would be easier for a person to deliver a punch with his left hand to the right side of Mayella's face) is so weak. Basically, Atticus is trying to establish a reasonable doubt in the minds of the jury that Tom is guilty. If not for the racism of the all-white jury, this almost certainly would have been enough to secure Tom's acquittal.