Atticus spends a great deal of time discussing Miss Ewell's injuries. What does he seem to want to reveal? The Ewell's are "poor...

Prior to discussing the injuries sustained by Miss Mayella Ewell during the alleged altercation, Atticus had Tom Robinson stand so that Mayella could "have a good long look" at him.  Scout's reaction to Robinson's physical appearance presumably mirrored that of others in the courtroom who did not know Robinson; she was shocked to see that Robinson

"...looked oddly off balance, but it was not from the way he was standing.  His left arm was fully twelve inches shorter than is right, and hung dead at his side.  It ended in a small shriveled hand and from as far away as the balcony I could see that it was no use to him."

Atticus used this opportunity to provide the jury, as well as everyone else present, with the visual information that should have led them to understand that Tom Robinson, with only one functioning arm, would have been unable to choke Mayella Ewell.  This fact should have raised sufficient reasonable doubt to have led to Robinson's acquittal, especially considering Bob Ewell's testimony stating that fingermarks were found "all around" Mayella's throat.  Robinson's disability clearly revealed his innocence.