Responding to Mr. Gilmer's questions, Heck recounts what happened the night Mayella was attacked. He said Bob Ewell called him, saying someone raped his girl. Heck goes to the Ewell house, finds Mayella beat up on the floor, and helps her to wash her face. He then asks her who beat her up and she said it was Tom Robinson.
Then, Atticus asks Heck if he called a doctor. Heck says no one called a doctor. Then Atticus asks Heck to describe Mayella's injuries. Heck says Mayella was hurt on the left side of her face. Then Atticus asks if it was her left or his left. Heck corrects himself and is then positive that it was the right side of Mayella's face that was banged up. Heck also says there were bruises on her arms and finger marks on her throat.
This was key evidence. Questioning Bob Ewell, Atticus shows the court that Bob is left-handed (by asking him to write his name). In Chapter 18, after questioning Mayella, Atticus asks Tom to stand up, revealing to the court Tom's mangled left hand. A left-handed man is much more likely to inflict damage to the right side of his victim's face. Atticus was making the case that Bob Ewell is more likely to have beaten Mayella than Tom because Bob was left-handed and Tom's left arm was a foot shorter than his right arm.