The three back stance punches in Hwa-Rang are an odd feature. The set appears in no Pinan/Heian kata. If they were intended to be used as punches, then a front stance would be more powerful. It's noteworthy that Hwa-Rang is an asymmetric pattern, meaning that presumably all three punches are necessary.
Hwa-Rang was made in the mid 1950s when the nascent Ch'ang Hon Taekwon-Do still used Shotokan karate chambers [1]. This means the low block was performed with the blocking tool traveling outside the reaction arm. One application of this chamber is a parry-pass to get outside the opponent's left arm.
Low parry pass. Image source: Traditional Taekwondo Ramblings |
We also move to a 90-degree angle from our opponent, represented by the 90-degree turn in the pattern.
From here, the "punch" could be simply a strike to the rib cage or side of the face. But we could also use it to grab the back of the opponent's shirt as you pull their arm with your reaction hand. We will be forcing this arm behind the opponent's back.
Step forward and pull in the back of the opponent's shirt (to prevent them from turning around) while forcing their arm behind them (the second "punch"). Why do this as a back hand punch? We are keeping the opponent's body close to ours (pulling them towards our front hip) for better control.
For the third and final punch we create a rear wrist lock. Grab the opponent's wrist with your right fist. Step forward and compress their backhand towards their head (the third and final "punch"), while maintaining control of their elbow with your left fist.
Some examples of a rear gooseneck lock, two pulled from the Encyclopedia of Taekwon-Do. Source for left image: Sabeel Combatives |
This lock is not very powerful, so we switch into the X-fist pressing block. One application of this technique is a "double wrist lock" or "kimura". Slip your left arm over and under the opponent's left arm and grab your own right wrist. Turn counter-clockwise and force the opponent downwards with the X-fist pressing block. This technique is powerful and can damage the opponent's shoulder.
Example of a double wrist lock, which may be used to drive the opponent to the ground. Source: Sidewinder Jiu Jitsu | |
[1] You may find evidence of this in Gen Choi's 1959 book Taekwon-Do Teaching Manual