Regular Flex. The ultimate in moderate swing speed performance for seasoned players, the luxuriously crafted XXIO Prime collection is in a master class all its own. Exquisitely designed to shine above the rest, XXIO Prime combines exalted looks with advanced ball speed technologies and exotically lightweight materials for a new pinnacle of high balance point golf clubs. On new XXIO Prime Irons, a soft/rigid structure combines a flex zone in the lower body with a rigid zone in the rear of the body for enhanced energy transfer in the lower half of the face.