Good question. I only remember a few things from past public speaking seminars, etc. I think if you google "body language public speaking" you'll find info. "Fig leaf public speaking" will tell you something about the posture I was speaking about. My cursory search found that hands behind your back aren't any better (according to the experts) but I think that posture was more common in the 60s and prior.
Either way, look at the clip with him and his shoulders bowed and hands cupped in front of his body. Freeze the frame and think of what it tells you. Your mind subconsciously picks up on cues like this.