Does anyone know how this works?

What I mean is how effective is it? And does the effect stack turn after turn, or does it only effect the current round its in. This passive is one I see occassionally against other people but since I am unsure of how it works I've been avoiding it just to keep my clone. But I'm in a position that I am trying to maximize my buffs and debuff my opponents as well. So if this is something that is effective I would like to use it.