I'm confused about what Haunting Guise's new passive does

So, quoting what the passive is on PBE: "Unique Passive - Essence Drain: While damaging champions, increase the damage you deal by 10% (max 2% per second)." To me, the wording on this is confusing as hell. How can you be increasing your damage dealt by 10%, but maxing at 2% per second? It's like saying distance and velocity as the same units. Could someone explain how exactly this passive works, and even better, can someone figure out a less confusing way of stating it, pending that I'm not the only one confused by this. Edit : I know what the item is designed for. My confusion is in how it works.
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