A device for controlling the breakover rate of a seatback in response to a dynamic event (regulating the breakover velocity so as to achieve optimal velocity differential to the impact velocity of a passenger's head impacting with the seatback) is attachable to the seatback and to the seat frame; when a breakover event is triggered, the passage of a head injury criterion (HIC) pin through a controlled traveling slot is impeded by energy absorbing materials selected, e.g. for material composition and structure, to restrict the breakover velocity of the seatback to optimize velocity differential between the breakover velocity and the impact velocity by engaging the HIC pin and deforming at a predetermined deformation rate.