NRG core's are lower density Polypro Core's. These 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 1.4lbs PCF. 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. To add more flex to the core some models add an ISS Stringer so you can add a more flexible or stiffer stringer & change the flex to suit the water temperature or your flex preferences.