To be honest I think it was more of the physcological things. The physiological part comes with the fact that when someone that seems dominant in an abusive relationship, the one being "dominated" you tend to think you are weaker, and that any attempt to get out or even getting out will not help. They saw more slaves getting captured rather than actually escaping, so add that to the fact that they tell you if you escape "we will do", fill in the blank. There was also the fact that white slave owners had guns, which might make it seem like your powerless.
Explanation:
BTW if i were them I would've used the term strength in numbers, maybe not in all cases would it work, but if executed right it could most of the time
