NRG core's are lower density Polypro Core's. This Low density PP Core's designed to have the lightweight strength & stiffness of Polypro core but with more flex like a PE Core. NRG Core is 1.5lbs PCF (Per Cubic Foot) This means they are lighter than PP as the Polypro Core cells are larger & less dense. This is the choice of core's if you want a more flex for cold water or a performance board that has more flex. This core is also great for kids or women as it is much easier to control.