From my experience, you can't be standing on it- just on the tiles next to it. Also make sure no enemies are aggro to you at all. That's worked in my experience
ok nvm I figured it out. Once a girl is helpless you gotta click the shackles on the bottom right and then click her, so instead of attacking her it'll capture her. Not super intuitive (in my opinion)
noob question, how do I do it? I did it once by walking into one but any other time I either trade places or accidentally defeat them :( can't really find anything in the game about the controls for it either but I could be wrong