Evidently it was by this ruse that Joe meant to take Ben into his household and away from his other brothers, leaving them none the wiser.
At this point, though, Joe's ruse of making himself strange unto his brothers finally falls through. When Judah offers himself in Ben's place, Joe can't take it anymore and loses it. He reveals himself to all of his brothers, declares them forgiven, and sends them back again to Canaan to retrieve their father.
Usually I have something snarky and sarcastic to say, but these chapters are really quite moving in a positive way. Good story.