Leech curses drain one opponent. Following each kill, the leech % resets, and %'s does not carry across opponents. You can drain 6%, rising to 12% of an opponent, and upon fighting a new opponent, this drain restarts at only 6%, to re-rise in time to 12% again. Player boosts of 2%, rising to 8%, do carry across, however, providing that the player is supplied with more drains to sustain the boost.