go reread chapter 3-4, that’s where the flashback takes place - basically yesol did not remember what happened the night before he woke up naked at hanlim’s place, so he was left wondering if something actually happened between them or not (the audience didn’t know either), turns out they did hook up, and while kissing for the first time hanlim bit yesol’s lips... fast forward to the present where they are in the alleyway, hanlim bites him again on the lip and it reminds him of that first kiss, so that makes yesol remember everything from that night

I DONT GET IT !!! CAN SOMEONE EXPLAINNN !?!!?!