You can improve and maximize some of your Swords' stats in the Refinery (錬結) menu.
To do this, you use other unwanted or duplicate Swords as material. Depending on which Swords you use as a material, you can boost the Impact, Leadership, Mobility and/or Impulse stat of a Sword.
You can select up to 5 Swords in which to refine another Sword with. Level of a material Sword does not effect the stat bonus.
It is recommended to only start refining your swords AFTER they have upgraded (特, toku). If you refine a sword before they Upgrade (Upgrade usually occurs at Level 20 or 25) you will lose all stat bonuses when they do.
It is recommended to ONLY refine a Sword who has not yet Upgraded if you are having difficulty leveling said Sword.