What does Atticus prove about Mayella's injuries during his cross-examination of Heck Tate?

In his cross examination of Heck Tate, Atticus proves that Mayella's beating, and the marks all located on the right side of her body could only have been inflicted by someone who is left-handed. He then goes onto reveal Tom Robinson's useless left arm. Thus, he proves Tom could not have beaten Mayella.

In conclusion, Tom did not beat Mayella.