nak bop sul are simply Falling Techniques. I just sounded so much smarter and fancier by using the Korean term. Put the term in your deck of terminology flash cards and sound smart like me! ;)
Falling is one of my favorite niche topics to work on. Everyone can and should be able to do it with a little practice. Of all the martial skills that apply to life outside of the dojang, this is among the most important. Everyone slips and falls, but not everyone takes the fall the same way. On the way back from lunch, I slipped on a wet step and recovered by going with the fall and dropping into a really low horse stance. Looked much better than me cracking my melon on the step behind me.
Most people fight the fall, struggle for balance, or vainly throw their hands out to catch themselves. How many of these people would willingly catch a 150 pound bag thrown to them? This is what you're doing when you fall on your hands.
When I teach my kids to fall, we use the big cushy mats, we have the wedge mats that they can roll down for backwards fall, and we use exercise balls to help them learn to roll. Kids who come in with gymnastics training have a little tougher time since they want to tumble. "Different skill" as a Master I know would say. Generally, kids love the falling class, and they love to line up at the end and I toss them with O-Goshi or Osoto gari. (Bad kids get Tomoe Nage :) ) They are young and fearless and, for some reason, trust that I have their best interests in mind.
My adults do not line up quite so readily. Why not? Well, they've already fallen a few times in their life. They've learned that falling hurts and they aren't really anxious to do it. So when I grab them, they tense up, they hold onto me, they even sometimes squeal in anticipation of what they perceive to be their imminent doom. Even after a successful toss and landing, they are not eager to repeat it. I remind my students that fighting the fall just makes it worse.
I believe falls should be practiced regularly to help keep the motion smooth, the neck muscles strong and confidence high. I really like this drill as a quick way to practice all of the falls:
Now what are you waiting for? Get off your seat and start nak bopping!