Source: Complete Champion
You can use your knowledge to exploit your foes' weaknesses and overcome their strengths.
Prerequisite: Knowledge (any) 5 ranks.
Benefit: Upon selecting this feat, you immediately add one Knowledge skill of your choice to your list of class skills. Thereafter, you treat that skill as a class skill, regardless of which class you are advancing in. Whenever you fight a creature, you can make a Knowledge check based on its type, provided that you have at least one rank in the appropriate Knowledge skill.
You then receive an insight bonus on attack rolls and damage rolls against that creature type for the remainder of the combat. The amount of the bonus depends on your Knowledge check result, as given on the following table.
|15 or below||+1|
|36 or higher||+5|
You can make only one Knowledge check per creature type per combat. If you fight creatures of multiple types during the same combat, you can make one Knowledge check per type, thereby possibly gaining different bonuses against different opponents.
This benefit is an extraordinary ability.