We aren't actually told how Judith manages to gain the red ear during the husking bee in Chapter 13 of this great novel. All that she says when asked is that she has "her methods" of making sure she gains the husking bee. What is ironic about this conversation at the beginning of this Chapter is that by the end of the Chapter, Judith has actually forced John to propose and so she doesn't need the red ear, so she chooses to embarrass Kit by throwing it to William. This embarrassment has just been made worse because Kit has just been proposed to by William:
Without waiting for an answer, she tossed it straight across the circle into William's hands. There were a few quick giggles, a hush of curiosity. Kit sat helpless, her cheeks on fire, and then the laughter and the cheering left her giddy as William stepped resolutely forward to claim his forfeit.
Obviously the significance of the red ear is something to do with love and that it will indicate who will marry next, or something like that, placing more pressure on Kit to make a decision about whether she will marry William Ashby or not.