The character has significant experience with two-weapon fighting, gaining +2.5% melee critical chance, as well as dealing 7 bleeding damage over 5 seconds. The bleeding damage does not stack with itself or any other damage over time effects.
If the character with Dual-Weapon Expert has a self-inflicted damage sustainable activated (Aura of Pain, Blood Thirst and/or The Tainted Blade), Dual-Weapon Expert begins to deal damage over time to the character. The appropriate icon appears over the ability bar. An unofficial fix for PC is available here.
The damage is supposed to scale with the character level but does not.