Hweh Juhn Nak Bub is a Rolling Fall.
This falling principle revolves around converting the force of a vertical fall into a horizontal roll. This is accomplished by ensuring that the point of contact between the ground and the practitioners body is curved sufficiently to translate the kinetic energy into its new direction.
The practitioner turns their head to the side, and allows their hands to touch the ground first. The hands guide the fall to the shoulder, and the roll occurs accross the curve of the back, from soulder down to the opposite hip. As the practitioner comes up from the roll, he slaps the mat, negating any remaining forward kinetic energy.
Beginners place their left knee on the mat, keeping their right leg bent at 90°. They hold the inside of the right foot with the right hand, and gently fall forwards onto their shoulder, looking to the side. Holding on to the right foot keeps them in the right position, when otherwise the instinct is to allow yourself to roll onto your side. Intermediate students execute the roll from a standing position, first touching their hands and allowing that to guide the fall onto their shoulder. Advanced practitioners leap into the roll, covering a great deal of distance, both vertically and horizontally. This last method is also known as a Cat Roll.
The Physics of Martial Arts - Kinematics demonstrated by a Hapkido Dojang