In the trial scene in Part II of To Kill a Mockingbird, Bob Ewell's left-handedness is the smoking gun that implicates him in the beating of Mayella. Atticus gets Bob to write his name, and the jury notices that he's left-handed. Atticus says that since Mayella's bruises were on the right side of her face and that she was choked with two sets of good hands around her gullet, Bob Ewell is her attacker and not Tom.
Atticus' clever tactics should lead to a not guilty verdict from the jury, since Tom's left hand has been crushed in a machine. But, alas, racism makes the jury blind to the facts.
The incident in which Bob Ewell unwittingly signs his name with his left-hand is, indeed, the incriminating evidence that Atticus is seeking since as the previous post explains Tom Robinson could not have made the marks on Mayella. In addition to being incriminated as a witness, Bob Ewell is somewhat baffled since he does sense that he has somehow been tricked by Atticus Finch. Knowing that many in the courtroom already scorn him as "white trash," Ewell feels great resentment toward Atticus after this incident because he has tried to fraudulently boost his position in Maycomb society by implicating a black man, and now failed.
This incident precipitates Ewell's remarks that he will make Atticus pay for what he has done. Ewell spits at Atticus and threatens him one day after the trial is over as his hatred is projected onto Atticus who kept him from seeming worthy when he testified.