Otherwise referred to as a martingale, the undercheck is designed for those horses who have a tendency to throw their heads up. When properly fitted, an undercheck will prevent the horse from getting his head higher than a normal position, rather than forcing him to carry it lower.
This model is known as a "ring" or "running" undercheck/martingale. The buckle end is looped around the girth.
The reins/lines are fed through the rings on the neck strap and it provides less downward pressure than a standing martingale design. The amount of pressure can be adjusted somewhat by the driver through the amount of contact applied to the reins.