A jawbreaker is fitted under the horse's jaw. The shaped piece sits under the chin, and the bars are fed through the rings of the bit. The reins (lines) attach to the rings of the jawbreaker, effectively giving the driver more leverage for horses that lug.
This model is designed for horses that lug to one side. The longer bar should be positioned to the opposite side that the horse leans, thereby giving a wider angle to create straighter tracking.