37. Choong-Jang first spin and slide back

The set is:

• 5: R.F.S. left two-finger strike
• 6: In place, R.F.S. front backfist strike
• 7: L.F.S. rising block
• 8: R.F.S. middle punch
• 9: Moving right leg, turn 360-degrees CCW and then slide backwards into R.B.S. forearm guarding block

The original set in Woo-Nam is similar, except a high punch is used and the forearm guarding block is in fixed stance. 

I emphasize the slide because it's often ignored in performances. The second spin later in the pattern has no slide. The canonical application for the slide is that you are dodging an attack, but why the spin beforehand?

Starting with the two-finger poke, we should be pulling in the opponent's left arm. Use the supporting arm of the front backfist strike to hit down on the opponent's elbow. Then you may use the strike itself as more of an uppercut to the opponent's jaw. We follow with the rising block as a quick second strike.

Source: Traditional Taekwondo Ramblings

Alternatively, you could use the rising block to raise the opponent's left arm from underneath, or to attack their outer-elbow. Whichever option you choose, grab the opponent's left arm and pull it with your left fist as you punch the opponent in the ribs with the middle punch. You could also punch the jaw (as in Woo-Nam). It doesn't really matter; what's important is that you end up outside the opponent's left arm.

Now for the spin: we use this to put the opponent into a waki gatame ("armpit hold"). By spinning into the forearm guarding block, we rotate the opponent's arm and use the back arm to place downward pressure on the back of their elbow. The front arm both twists and lifts the opponent's forearm.

Pull and punch to waki gatame. Notice how the tori steps with their right leg, although he doesn't do a full spin in this case. Source: TRITAC martial arts

Google "waki gatame" for more examples.

From here the meaning of the slide is obvious (they even talk about it in the linked video): it's a way of breaking the arm, or at least forcing the opponent to the ground to avoid their arm being broken. A more extreme version of this is to sit down on the opponent as you lean your weight backwards.