-Unmastered: Drain some of the damage dealt to heal yourself.
-Mastered: Drain all of damage dealt to heal yourself. +30% zan boost (unconfirmed if unmastered gives same)
-General Description: Very useful, as the target may not notice that you're draining their health. Makes it far harder for them to kill you! :D
Master it by slashing people or NPCs with Shikai on.