Castlevania: Portrait of Ruin

If you equip the ring you get more SP for each enemy defeated using a sub-weapon.

Most subwepons/skill has a number of mastery points needed for Jonathan to master it, thus increasing its power and abilities (ans look).SP is the number of mastery points you recieve by killing a enemy with a subweapon/skill.
The mastery ring doubles the ammount of SP you can get for killing a enemy with a subweapon/skill.

Ex:The Shuriken, if i'm not mistaken, needs 300 SP to be Mastered.Once Mastered it will increase power and will double the ammount of shurikens to be used (4).

if before Mastrered, you kill a zombie with the shurikem you will get 1 SP and with the Master Ring Equiped,2

Also, the subwepons/skill doesn't needs to be mastered to increase strength