Ahp Nak Bub is a Front Fall.
One of the first falls taught in Kuk Sool Won™, Ahp Nak Bup involves falling face first from a position in the air in which the student's body is horozontal. The student lands on the balls of his feet, and on the plane between wrist and elbow. Hands are flat down, fingers spread apart. Head is turned to one side.
Beginners are taught to fall forward from a kneeling position, so that they can slap the mat with the appropriate section of their arms, and so that they can get in the habbit of turning their heads to the side. Once that is mastered, they are taught to shoot their legs out behind them as soon as they've slapped the mat, allowing them to get used to supporting their body from the forearms to the balls of their feet.
Once this is mastered, the student can generally move on to a proper Ahp Nak Bub from a squatting position, and then from a standing position, and ultimately from elevated heights.