about the dark harvest keystone

I play {{champion:35}} jungle with dark harvest but I don't understand how it woks yet. is it like you collect some souls from your jungle clears and when you gank your first damage to champion uses them all? meaning it resets the number? or is it like they stack until you die? since there is no cooldown mentioned in it I think it's the first way but... if you use all the souls on your first damage to a champion why does champion kills give you souls? for your next gank? please give me some explanations thanks
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