That was the first time Bu Yeon and Nak Su were both active at the same time. Bu Yeon used Nak Su's ability to…
You didn't explain why not so I'll guess: because maybe those are just special effects?
If so, whenever AoS is used, they do see the "special effects". It isn't farfetched that they can also see the light. If I remember correctly, in the mirror episode, Uk explained that there a strong bright light inside the cave and the next thing he knew the mirror broke and Mu Deok escaped.
Also, Uk hesitated. He was processing what he witnessed and what Mu Deok told him. Eventually he looked like he understood what the light meant and took the risk in embracing her because he knew she won't absorb him. Remember in the beginning of the episode, Mu Deok realized that Jang Uk wasn't fazed he lost rather he was replaying his fight in his mind. I think it's what Uk did when he hesitated.
1. The Queen is actually a Shaman. The real Queen is in the Shaman's body - wherever she's locked up.
2. Nak Su's assassinations were to kill the "useless bodies" after the "VIP soul" shifted into a new one. She was not killing mages because of revenge or hatred. She was just following orders, biding her time, waiting for an opportunity to find the Four great families.
As Park said, she was used her entire life while believing the Four great families killed his father for no reason. (But we know Nak Su, Uk, and Yul, are also already aware of the facts; and Nak Su and Uk are waiting for an opportunity.)
3. Bu Yeon is definitely protecting Nak Su. Not only because she will also die if she doesn't protect the "body" but because she needs her. Bu Yeon used Nak Su's ability to absorb energy (since she's a shifter) and she used her power to purify the energy. Bu Yeon is keeping Nak Su alive, preventing her from going wild and petrifying.
4. If Nak Su's energy is boosted, she overpowers Bu Yeon's. When Bu Yeon's power is boosted (or activated) she overpowers Nak Su ("Mu Deok" temporarily goes blind). This is probably one reason why Bu Yeon is not showing herself frequently, Jang Uk and Nak Su may figure it out, or others will.
5. The jade pendants are important. It will allow Jang Uk and "Mu Deok" to share their energy. Maybe Jang Uk sharing his energy is the key to making Nak Su's and Bu Yeon's energies to work in tandem instead of trying to overpower each other.
I can't think of a good analogy but let's say "transformer" (in electrical) or a "bridge". Maybe Nak Su's and Bu Yeon's energies are in contrast with each other (Yin and Yang) but in reality their energies complement each other. To create that "connection", Jang Uk's energy is needed … and Jang Uk can only share his energy through the pendant.
And maybe, whatever power Jang Uk has, it will only be unlocked with the combined powers of Nak Su and Bu Yeon. We go back to, again, Yin and Yang, two opposing energies that actually complements each other.
Just wondering and curious........Only Uk, Yul, master Lee and the audience know that Naksu is inside Mu Doek's…
Their country is not trapped in hierarchical ranks. Their position are simply roles.
In the modern times, a good analogy would be an employer who separates a person's pay from their ranks and roles. Most companies today give the same level of salary based on ranks. They mix rank and position. However, there are a few companies which separates these two. So say, 2 people with the rank of supervisor are paid differently based on their performance and role.
In Daeho, Mu Deok's rank is a servant, her role is a maid. The see ranks are nothing but hierarchical order. And no matter what one's role is, they are all equals. They are all integral part of the machine that makes the nation work.
Did I understand correctly that the jade pendants themselves attract each other (like magnets) and not the "souls"…
The two jade pendants are filled with the energy of Yin and Yang. Two opposite energies that actually complements each other.
So technically, it is not the pendants that is attracting each other but the energies inside a person. If one of the pendant is activated, the Yin or Yang energy in it dominates that persons holding them, which they makes them to look for each other because Yin and Yang complements each other.
That was the first time Bu Yeon and Nak Su were both active at the same time. Bu Yeon used Nak Su's ability to absorb energy while at the same time she also used her own powers to make the transfer pure.
Notice the difference in special effects.
1: A shifter absorbing another person's energy, the sfx is dark energy or shadow. 2: Bu Yeon purifying the energy being absorbed by Nak Su (as a shifter) is light, intense light.
I think Jang Uk also saw it and this is the reason why he embraced her. He understood the difference. He understood the meaning of the intense light.
Secondly, remember during the mirror episode? What broke it was Bu Yeon's power, the intense light (or energy).
Third, in the preview, Mu Deok's temporarily blind again. The effect of the intense light did not show up immediately (like in the mirror episode) because she just absorbed an energy. As Nak Su, her own power is much stronger than Bu Yeon at that moment. But once that energy she absorbed "calmed down" (for lack of a better term), Bu Yeon, having activated herself, overpowered Nak Su thus Mu Deok (the body) turned blind temporarily again.
Once Bu Yeon's power "calms down" Nak Su and her will be on equal footing again, cancelling each other … which would mean since Nak Su is the one with control over the body, Mu Deok will get her sight back again.
1 
https://i.ibb.co/5YZg2Gk/its-beautiful-now-butt-ad.png ( https://ibb.co/YDgFbPX )
https://i.postimg.cc/903cQJnK/its-beautiful-now-butt-ad.png ( https://postimg.cc/sQJkn9y4 )
If so, whenever AoS is used, they do see the "special effects". It isn't farfetched that they can also see the light. If I remember correctly, in the mirror episode, Uk explained that there a strong bright light inside the cave and the next thing he knew the mirror broke and Mu Deok escaped.
Also, Uk hesitated. He was processing what he witnessed and what Mu Deok told him. Eventually he looked like he understood what the light meant and took the risk in embracing her because he knew she won't absorb him. Remember in the beginning of the episode, Mu Deok realized that Jang Uk wasn't fazed he lost rather he was replaying his fight in his mind. I think it's what Uk did when he hesitated.
2. Nak Su's assassinations were to kill the "useless bodies" after the "VIP soul" shifted into a new one. She was not killing mages because of revenge or hatred. She was just following orders, biding her time, waiting for an opportunity to find the Four great families.
As Park said, she was used her entire life while believing the Four great families killed his father for no reason. (But we know Nak Su, Uk, and Yul, are also already aware of the facts; and Nak Su and Uk are waiting for an opportunity.)
3. Bu Yeon is definitely protecting Nak Su. Not only because she will also die if she doesn't protect the "body" but because she needs her. Bu Yeon used Nak Su's ability to absorb energy (since she's a shifter) and she used her power to purify the energy. Bu Yeon is keeping Nak Su alive, preventing her from going wild and petrifying.
4. If Nak Su's energy is boosted, she overpowers Bu Yeon's. When Bu Yeon's power is boosted (or activated) she overpowers Nak Su ("Mu Deok" temporarily goes blind). This is probably one reason why Bu Yeon is not showing herself frequently, Jang Uk and Nak Su may figure it out, or others will.
5. The jade pendants are important. It will allow Jang Uk and "Mu Deok" to share their energy. Maybe Jang Uk sharing his energy is the key to making Nak Su's and Bu Yeon's energies to work in tandem instead of trying to overpower each other.
I can't think of a good analogy but let's say "transformer" (in electrical) or a "bridge". Maybe Nak Su's and Bu Yeon's energies are in contrast with each other (Yin and Yang) but in reality their energies complement each other. To create that "connection", Jang Uk's energy is needed … and Jang Uk can only share his energy through the pendant.
And maybe, whatever power Jang Uk has, it will only be unlocked with the combined powers of Nak Su and Bu Yeon. We go back to, again, Yin and Yang, two opposing energies that actually complements each other.
In the modern times, a good analogy would be an employer who separates a person's pay from their ranks and roles. Most companies today give the same level of salary based on ranks. They mix rank and position. However, there are a few companies which separates these two. So say, 2 people with the rank of supervisor are paid differently based on their performance and role.
In Daeho, Mu Deok's rank is a servant, her role is a maid. The see ranks are nothing but hierarchical order. And no matter what one's role is, they are all equals. They are all integral part of the machine that makes the nation work.
So technically, it is not the pendants that is attracting each other but the energies inside a person. If one of the pendant is activated, the Yin or Yang energy in it dominates that persons holding them, which they makes them to look for each other because Yin and Yang complements each other.
Notice the difference in special effects.
1: A shifter absorbing another person's energy, the sfx is dark energy or shadow.
2: Bu Yeon purifying the energy being absorbed by Nak Su (as a shifter) is light, intense light.
I think Jang Uk also saw it and this is the reason why he embraced her. He understood the difference. He understood the meaning of the intense light.
Secondly, remember during the mirror episode? What broke it was Bu Yeon's power, the intense light (or energy).
Third, in the preview, Mu Deok's temporarily blind again. The effect of the intense light did not show up immediately (like in the mirror episode) because she just absorbed an energy. As Nak Su, her own power is much stronger than Bu Yeon at that moment. But once that energy she absorbed "calmed down" (for lack of a better term), Bu Yeon, having activated herself, overpowered Nak Su thus Mu Deok (the body) turned blind temporarily again.
Once Bu Yeon's power "calms down" Nak Su and her will be on equal footing again, cancelling each other … which would mean since Nak Su is the one with control over the body, Mu Deok will get her sight back again.
Weirdest ad I've seen from Korea. hahahah
I can't wait for episode 6, I wonder why Yu Jin cried.
Rollercoaster ride now …