What evidence does Atticus bring out that discredits Bob Ewell's story?

There are two pieces of evidence that discredit Bob Ewell
completely and they have to work together. First, Atticus shows that Bob Ewell is
left-handed, and Bob tries to show that he can use both hands, but it's too late. Then,
when he asks Tom to rise after Mayella pointed him out, the courtroom sees the other
necessary piece of evidence: Tom's left arm is 12 inches shorter than his
right.

This proves it would have been impossible to leave
the injuries on Mayella that were there. It also leaves the question on the table, if
not Tom who? The fact that Bob was left-handed proved that it was possible he beat his
daughter because the person who hurt her led exclusively with his
left.