At 5th level, the duelist gains the ability to strike precisely with a piercing weapon, gaining 1d6 piercing damage. This only works against enemies vulnerable to critical hits. Precise Strike is automatically in effect when the duelist has no off-hand weapon or shield. At 10th level, the damage increases to 2d6.
Contrary to the description, this ability does work against opponents who are immune to critical hits. The bonus damage is not multiplied on a critical hit. It will not work if you have a shield or a second weapon equipped, but it does work with two handed piercing weapons (e.g. you can make a spear duelist).