What I understand is that the boy is fantasizing on his girlfriend's mother. The girl is mad about it but to please her boyfriend, she decides to merge with her mother's body. For some reason, we find out at the end that it was actually the mother in control of it.
Belated reply for a summer but what happened was: The daughter leaves the room and comes back with her head merged with her mother's body. By the end you find out she thought her mom was always unaware of the time they are merged, but she's really seen all of it but never said anything and also has the same power. (This isn't the first time she's borrowed her body) So she teases her/teaches her a lesson by doing it with the boyfriend way more aggressively than the daughter would have done, knowing her daughter is aware of everything happening. She tells her as you get better at the technique you're able to take over control of the merged body like she did with the daughter.