How does bone curse work?

[Seltin]

I think I understand the ability itself, but it what triggers the curse? I know Harvest does, as it says so in the tooltip, but can I cast bone curse then cast rip blood and will rip blood trigger the curse, or is it only certain abilities?

[Arqwart]

Any direct hits on a cursed target will activate the curse's damage, be it you, an ally, or a summon.