Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

I got it! I figured out why they stripped Ruby to his underwear right away while they were interrogating him on the 5th floor. It was to keep him from running off; running away. 

Obviously, he wasn't going to get away. But, he wasn't in his right mind, and they had to make sure that he stayed put. They had him there probably close to an hour before the televised spectacle occurred. If he ran off; if he made a scene; if he drew attention to himself by bolting, it would have endangered the ruse. It would have been an incident that had to be reported, written up with a time. Someone who wasn't in on it might have seen him. It would have involved noise and someone might have heard it. They had to restrain him, but, they didn't want to tie him up, shackle him, keep him handcuffed, etc. That's because he was Jack Ruby: friend of the Dallas Police. The record shows that after his arrest they all called him "Jack." No one addressed him any other way. It was always Jack, Jack, Jack. They wanted to keep it friendly and gentle. So, they kept him in his underwear because it was like a very, very, very gentle physical restraint.