It’s a shocking moment during the Mugen Train arc, but why did Rengoku choke the girl?
Rengoku had to choke the girl because she was about to destroy his spiritual core. His body instinctually protects itself from danger while he’s asleep.
The two are at a stalemate, as she does not have the strength to free herself and Rengoku can not end a person’s life.
Rengoku’s unconcious body decides to choke the girl
Chapter #56 begins with a visual of a rope connecting Rengoku to a girl. Enmu, the Sleep Demon, makes rope with the power to enter the dreams of those who are bound by it.
The girl, looking to appease Enmu’s request to destroy Rengoku’s spiritual core, walks through the Flame Hashira’s dream. Enmu insists to the girl that if she follows his orders, she’ll be granted eternal sleep with her family members and loved ones.
She notices the “main self” version of Rengoku, who is training his younger brother, Senjuro. In an effort not to be noticed, she runs to the edge of Rengoku’s dream. We see no mention of Rengoku’s father, who quit being a swordsman.
The dreams that the Sleep Demon Enmu creates do not stretch out forever. They are circular, the girl explains, with the dreamer at its center. The outside of the dream is where the unconscious state exists, holding the dreamer’s spiritual core.
If the spiritual core is destroyed, the girl says, then the dreamer will become an empty shell of their former self. She finds “the wall” of his dream and attempts to pierce through to reach the unconscious realm.
The girl attempts to destroy Rengoku’s spirital core
She enters Rengoku’s unconscious realm. Hot and burning, the bright red spiritual core floats within its center. The girl raises her sharp object to stab the core. However, she is interrupted in real life by a sleep-walking Rengoku grabbing at her neck.
Usually, a narrator explains, people who have fallen under Enmu’s spell cannot move. Their physical and conscious bodies are completely severed while inside of the dream.
Enmu was being cautious and knew of a possibility that the Demon Slayers, despite succumbing to his spell, could sense his malevolence and break it.
Those who have lost their spiritual cores can not resist when they are in danger. Enmu commissions humans to enter the Slayers’ dreams and destroy their cores before real-life reactions can take place.
Yet, Rengoku is able to move, despite being under a spell that is meant to immobilize him. The girl who enters Rengoku’s dream gives off no evil aura, but the Flame Hashira’s body instinctually senses the destruction of its spiritual core.
The girl is unable to move in real life, but Rengoku’s body will not allow him to end a human’s life. This causes a stalemate.
So that’s the answer. Rengoku had to choke the girl because she was going to destroy his spiritual core. His body instinctually reacts to the danger and protects itself while Rengoku is still in the dream state.