6: Choong-Jang arm drag to rear naked choke

Here's an application straight out of a jujitsu class. It uses the set from Choong-Jang:

• Back stance low ridgehand guarding block (move 37/39)
• Stance shift into front stance nine-shape block (move 38/40)
• Chamber for move 41, i.e. crossing both arms across chest while moving backwards

The use of the first two movements are shown in the gif below.

Source: Relson Gracie Jiu-Jitsu Academy

Details:

• Use the guarding block chamber as a "wax off" motion against a push or attempted grab.
• Circle downwards, gripping the opponents wrist and triceps with the back and front palm of the guarding block respectively. (For the ridgehand guarding block, both palms face upwards)
• Pull to the side while stepping forward behind the opponent, getting to their back. 
• Use the upper arm of the nine-shape block to get a choke, while you use the lower arm to press on the opponent's lower back, breaking their structure. 
• You can then use both arms to do the choke as you drag your opponent backwards.

Using the lower arm of the nine-block to break the opponent's structure is not a trivial step. The rear naked choke is difficult to escape once balance is lost. Other tutorials recommend you use your knee to break the opponent's balance instead, such as this one by Stephan Kestling.

I admit that I am adding extra footwork to the form (stepping forward while you pull) and changing the nature of the nine-block somewhat. But I believe this is a good self-defense technique for any taekwondoin to know: arm drags and rear naked chokes are both grappling staples. The crossing of the arms (chamber for move 41) is how Stephan Kestling mimes the movement.

In Stephan Kestling's version of the rear naked choke, the knee is used to break the opponent's structure.
Source: Self Defense Tutorials

I also believe this to have a higher chance of working than the usual "joint break" application given for the nine-block, where the lower arm pulls down the opponent's wrist while the upper arm rises under their elbow. This may seem like a good application for this set on first glance, but it's unlikely to work against a resisting opponent because it relies on weak shoulder muscles. A better way to break with the nine-block is to overhook the opponent's arm and get a figure-four lock, shown in the bottom half of the below image. This is more stable and lets you use stronger muscles to break. I believe this is the use of the nine-block in Gae-Baek, but it does not fit the movements in Choong-Jang.

Top: "Joint break" application for the nine-block from the Encyclopedia of Taekwon-Do. I have doubts about this application. Bottom: A different way to break with the nine-block.
  Sources: Total Self Defense, Radek Scuri