In "To Kill a Mockingbird", why does Atticus ask Bob Ewell to write out his name? What does the jury see when he does this?(In Chapter 17 of 'To...

Atticus Finch makes Bob Ewell sign his name to prove in court that Ewell is left-handed. As Tom's own left hand has been useless ever since a farm accident a few years earlier, this proves that Mayella's aggressor is someone left-handed (such as her own father). For the bruises inflicted on her have been done by someone who had use of both his hands and who was in proximity at the time of the incident.

Atticus catches Ewell off guard, who does not see into what the lawyer is driving at until it is too late.