Isaac's Swearing on the Future?

Ephron, who's a hittite, offers a burial place as a gift. And abraham says, no, you know, you have to let me pay for it. And epron says he will. And so he has a good burial place for his wife. And then abraham decides that isaac needs a wife. He sends his eldest servant to mesopotamia to find a wife for isaac. There's a strange ritual that's performed. So it says in the story that the servant places his hand under abraham's thigh to to swear. But that isn't really what it means. It means that he places his hand, i don't know