Yes. It allows you to retain up to 40% of your total rage at all times.
However, although it might seem very, very cool to you, it isn't
If you have rage available, your mana regeneration is decresed based on the amount of rage you have at that point. So in essence, a warrior with bloodlust will regen mana very slowly without any means of boosting the out-of-combat mana regen, so it will be imperative for him to finish all fights with as much mana as possible -unless the player doesn't mind waiting 1-2 minutes (and probably pacing the room) for his mana pool to fill up.