The breeches, or trousers, held up by braces would have made it very difficult to slouch, giving Mr Darcy and Mr Bingley the upright stance we associate with men in the Regency period.
Do not confuse this with the seriously inconvenient condition known to the world as buck teeth (without which we would have no orthodontists, and no mortified adolescents with mouthfuls of rubber bands and wire braces).