In the late 1900's when dueling began to be frowned upon by the law, schedules to fight started to exclusively occur at night. In order to allow your opponent to see you in low light conditions, the combatants would hold lanterns in their rear hand. This tradition is why modern fencers hold one hand (fabulously) in the air behind them, or across the small of their back. The magicked crystal medium is the tradition-friendly interpretation of the dualist's lantern.