bales forming the edge of the sumo ring
Sumo term: the straw bales that define the boundary of the dohyō. Stepping outside these bales results in a loss.
The match was decided because the wrestler stepped over the edge bales.
Compound of 勝負 (shōbu, 'match') and 俵 (tawara, 'straw bale'), referring to the bales that determine the outcome of a sumo match.