The act of soul possession isn't that mystical; it's just a soul clashing with another soul, and whoever wakes up first wins. Success or failure, it's a fifty-fifty chance.
Of course, there are always those who like to add some flair, using esoteric methods and sacrificial rituals, like playing some background music during the process, with songs and dances, turning it into a festival. The method uses familiar tunes or suggestive ways to specifically awaken a personality, a tactic most commonly used by the saintesses of the Divine Teachings.
There are also more brutal and direct methods, such as those employed by Xuanmen, who take advantage of the vessel being in a deep sleep during a dream to sneak attack, or causing mental distress with illusions to break the psyche. Or they might just be blunt about it, breaking the hands and feet, peeling the skin off bit by bit, relying on pain to shatter the personality, easily seizing the dao body.
