You use your knowledge about the target to bypass its resistances and damage reduction.
When performing a studied song, designate one target affected by the song. Attempt an appropriate Knowledge check based on that target’s creature type as you perform the song. The DC for this check is equal to 20 + the creature’s CR based on its race and not including any class levels or template (a creature that is defined by class levels has an effective CR of 0 for this ability). If you succeed, your studied song ignores any elemental resistance or damage reduction the target has because of its race as well as any bonuses on saving throws against the song granted by the target’s race (such as the bonus from a dwarf’s hardy ability). Your studied song doesn’t ignore elemental resistance, damage reduction, or saving throw bonuses granted by other songs and effects. If you fail the Knowledge check, the song still has its normal effects.
MP Increase: +2 (a studied song costs 2 additional MP more than the actual MP cost of the song.) In addition, the perform check DC increases by +2.