After completing trial 5047, receives the augment "Enhances Saber Dance Effect." This enhancement adds 1% to the baseline double attack of Saber Dance per merit level. So at Saber Dance 5/5, you get 5% more Double Attacks.
The trial requires you to obtain 20,000 Experience Points in either Dynamis - Beaucedine or Dynamis - Tavnazia (the place where the base item and the -1 version drop, not so coincidentally).
Experience rings, Corsair's Roll, etc., count toward the objectives of this trial. I'm not sure about battle theories, etc., though!