I think the partition has two parts. One is made up of bamboo slits and other is translucent shoji paper.
It'd be correct to assume that only a silhouette could be seen with these two partition.
Both were down at the beginning but suddenly the shoji part rolled up with a "swoosh." This alerted Seunghyun and he reflexively covered Yuta's dick. With the paper partition up, the old geezers can see everything although they might need to squint a bit to see through the gaps in the bamboo slits.
Oh wow, thank you so much!!! That was very informative. It makes much more sense to me now - with such mechanism, rolling up part of the partition had to be done deliberately!
Can someone please explain how the screens/partitions worked or rather didn't work while they had sex in front of the group? I assumed that you could just see a silhouette but then the ML suddenly covered MC with the blanket and acted like the other people could directly see them? I thought the partitions might have fallen or something but it still isn't clear to me what actually happened.