You first have to take the "Cold Blood" skill perk. Which is silly, as it allowds you to purchase other cold blood enhancers, before you even obtain Cold Blood.. But its very self explanatory. Im not understanding how this is confusiong, unless maybe there is a language barrier? Like many RPGs, its a buff that activates 'On Kill'. Meaning, when you kill an enemy, you get the "Cold Blood" buff for 10 seconds (before any other enhancement perks you have invested into). If you have the UI enabled, you will see the buff icon. Again, you have to actually get the Cold Blood perk in that tree. Its one of only 3 or so available in the tree, until you invest more into "Cool" stat