User blog comment:E-113:Xi/The Neo Stellar Royale!!/@comment-2221250-20121201025046/@comment-4232858-20121202000341

I mean that the demon is rooted to his soul, so it cannot leave his body and can rarely (maybe only once) forcefully take control of the host's body and fight/kill the opponent, while causing extremely severe recoil damage to the host when it releases control.