How is Tom's handicap important to his case?Harper Lee's To Kill a Mockingbird

Tom Robinson has a withered left hand that has been
rendered useless.  Since the Ewells claim that Tom has tried to beat Mayella, grabbing
her by the neck.Mayella testifies,

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"He got me round the neck, cussin' me an' sayin'
dirt--I fought'n'hollered, but he had me round the neck.  He hit me agin an'
agin--"

Atticus Finch points
out Tom's handicap in order to disprove the possibility of Tom's having been able to
inflict the reported injury upon Mayella, and to discredit the testimony of the
witnesses.  For, it would be physically impossible for Tom to have choked Mayella and
the marks on Mayella's face reveal that the bruises were inflicted by a left-handed
person because they are on the right side.

With such proof
as this, a defendant should be acquitted of the charges.  However, Tom is not; thus, his
conviction is clearly made by the bias of the jury.