How did Tom help Muff Potter at the trial?

The defense attorney calls Thomas Sawyer to the witness stand. Puzzlement crosses the entire audience as Tom is administered the oath. Nervous and scared, Tom recounts the events of the night spent in the graveyard. "ŠMuff Potter fell, Injun Joe jumped with the knife and Š" says Tom; and with the sentence unfinished, Injun Joe tears through the courthouse, out the window, and disappears.