At 6th-level, Duelists can make an impressive, flashy attack with a piercing weapon that catches their foe off guard. This attack deals 2d6 additional points of piercing damage. There is no limit to the number of times per day the duelist may use this ability, but has a 30-second “cool down” between uses.
According to feats.2da the cooldown of this ability is actually set to 12 seconds, not 30.
Regardless the description, it could be used with all finesable weapons even when dual wielding.