Regardless of whether you believe that the girl is an evil spirit or a good spirit, how do you think her trap actually functions?

Out of all the events and ideas in the movie, this is the only one I'm unclear about. You can check out my long analysis of the whole movie here: https://www.themoviedb.org/movie/293670-gok-seong/discuss/590335749251413e3800a279

I will copy/paste my theories about the trap. I still don't have a concrete idea about what it does. Or, I can't put all the hints together to come up with a logical deduction. Maybe I'm missing some crucial hint.

There are a few possibilities as to how the trap might work:

Possibility 1) If someone goes past the flowers, the flowers wither. The person who moves past the flowers become impure or a sinner, and thereby they are vulnerable to being interacted with/possessed by the evil spirit. Jong-goo had already sinned when he killed forest shaman, this is why the evil spirit could touch his hand. When Jung-goo passes the trap, it has no effect on him because he was already a sinner, but him passing triggers the trap and as a result it cannot be triggered again when town shaman approaches the trap later.

Possibility 2) We see very early in the movie during the investigation of the first murder scene, Jung-goo notices withered flowers at the pig farmer Hueng-guk's house. One room in the farm had a nest-like bed which we know is used in death hex rituals where a possessed person sleeps in the bed while the shaman attempts to perform a ritual to kill the perceived evil spirit.

Based on the level of infection in the Hueng-guk's face, we know that the he had been possessed for some time. Cho, the ginseng farmer, was possibly a shaman who was called to exorcise Hueng-guk, but because Cho triggered the flower trap, the ritual backfired (we know from town shaman's words earlier that the hex is prone to backfiring). The backfire caused Hueng-guk to kill Cho, and later his family.

Possibility 3) The trap causes the possession to spread. The evil spirit wanted to infect the town shaman so that he could carry the infection out of the village and infect others beyond the village. She is upset when Jung-goo unintentionally ruins her plan by triggering the trap because all of the village is already almost dead and she has nobody else to infect/possess.

Possibility 4) The evil spirit was lying about the trap to keep Jung-soo with her for longer so that she could possess him. The possession possibly takes some time to complete. If the rooster crowed 3 times then she would have enough time to possess him. Watch how she draws closer and closer to him and touches him near the end.

Possibility 5) The flower in the archway helps the evil spirit create a barrier so that the shaman can't enter. This is why she is able to make him bleed/vomit earlier when he tries to pass. When Jung-goo passes the barrier, he breaks the protective barrier allowing the shaman to enter.

Please add your theories about how the trap might work. Maybe it's clear to you.