Yuhp Nak Bub is a Side Fall.
Yuhk Nak Bup is designed, as the name implies, to deal with a fall in which the practitioner lands on his side. The idea is to protect the head by tucking the chin and looking over the shoulder in the opposite direction of the fall, to protect the knees by keeping the legs separate, and most importantly to protect the body by slapping the mat prior to impact.
The beginner performs the fall by sitting down on the mat, and gripping his belt knot with the hand. Whichever hand he grips with dictates the direction of the fall. He then falls over to the side, slapping the mat with the opposite hand, turning his head away from the fall to look over his shoulder, and keeping his legs separated so that his knees don't hit one another. The shoulder he didn't fall on should not be allowed to touch the mat, to further protect the head. The leg facing the fall should be on its side, while the other should have the knee up, creating a separation between the knees.
At intermediate levels, the practitioner stands on the mat, and allows one leg to collapse. As an instructor once said, "Don't fall over like a tree! Go in stages." What he meant was to allow first the knee to touch the mat, and then the hip, and then to slap the mat, and then the shoulder touches.
At advanced levels, the practitioner can leap into the air and fall with no difficulty.