It does many things including lowering your max HP by 50%. He's using it with the Dusk Crown Ring (lowers max HP by another 50%) to get his HP so low that the health drain from Power Within and the health regen from Sanctus cancel each other out, which enables him to have Red Tearstone activated for the full duration of Power Within; aka Super Hyper Mode. More info on Curse status here: darksouls.wikidot.com/curse
Are you still there?
What does the cursed state does?
It does many things including lowering your max HP by 50%. He's using it with the Dusk Crown Ring (lowers max HP by another 50%) to get his HP so low that the health drain from Power Within and the health regen from Sanctus cancel each other out, which enables him to have Red Tearstone activated for the full duration of Power Within; aka Super Hyper Mode. More info on Curse status here: darksouls.wikidot.com/curse