Dee Nak Bub is a Back Fall.
As the name implies, this fall involves landing on your back. Ideally, the Kuk Sool Won™ should be able to fall from an intermediate height and roll into stance.
Beginners sit on the edge of a practice mat, and practice falling flat on their backs from a seated position. The objectives are to get the student in the practice of tucking his chin down to his chest, and to familiarize him with slapping the mat at the correct angle with his forearms. Like Ahp Nak Bup, the forearms make contact from wrist to elbow. Next, the student completes a back roll after impact. Specifically, as soon as the arms and lower back impact the mat, the student throws both his legs over one shoulder or the other, keeping his head tucked in, and rolls into stance. At intermediate levels, the student appraches Dee Nak Bup from a squat, and then from a standing position. Advanced practitioners should be able to execute this fall from a significant height.