so, if you're confused...which you probably are...let me explain... (i don't usually do ending explanations, but i believe this ending certainly warranted it, so here we go)
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to understand the ending, you first need to understand that jungkook is essentially crazy at this point. the trauma of the entire situation, plus mental stressors of guilt, increasing paranoia, and hallucinations near the end (potentially influenced by lack of sleep) shatter his mental stability. so if you're trying to read the ending as though he is sane, you're not reading it quite correctly.
you also need to understand that jungkook, in his shattered mental state, views all the hands together as a grouped identity. when he refers to "sukyeong", that is his mind translating what is before him - 6 hands that essentially spell out the name "sukyeong" - into one group identity.
when jimin's hand is separated from the rest, jungkook is confused because he then stops seeing jimin's hand as part of the whole (sukyeong) and instead sees it by itself (first as the character it represents, ㅅ, and then as an unknown identity).
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the second thing you need to know to understand the ending is a basic understanding of korean characters, so i am here to walk you through it!
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in the beginning scene, the table is set as such (each character shown below representing the hand in sign language, so character-wise, left to right would be jimin, hoseok, taehyung, etc.)
reading from left to right, we would pronounce this as "sukyeong" (숙영).
in the story itself, when jungkook refers to "sukyeong", it means that in his mind, he recognizes the combination of the six hands in that order as the name "sukyeong."
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when jungkook removes jimin's hand from the table, he looks back to no longer read "sukyeong" (숙영) but instead "ukyeong" (욱영). now, that first character (ㅇ) is technically not on the table, but according to conventional writing rules in hangul, you must start with a consonant, so to make "ukyeong" grammatically correct, i have written it with the ㅇ character in front.
thus, "sukyeong" becomes (at least phonetically) "ukyeong."
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finally in this scene, jungkook walks around the table, thus flipping his view 180 degrees, and these characters can all be read upside down as separate characters (ㅕ upside down becomesㅑ and so on), so what used to be "ukyeong" now reads "yangno."
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if you understood even half of this, then you are a very smart reader indeed :)
additionally, i wanted to note that it is unclear to me as to whether or not "sukyeong" is a valid name in regards to korean naming conventions, as i am not a native speaker, so i apologize (but i really needed something with 6 characters so ;;;)
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i hope this helps clear up a little bit (and if it didn't, i'm sure it just created more questions)...you're welcome...
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as always, thanks for reading! especially this dark and gruesome ride into the horror genre. it's been fun! see you next time :)