...someone correct me if I'm wrong.
That video shows a stance training exercise going from sitting on crossed legs on one side to the other. If you look closely, as they do 180 degree turns with their bodies, each foot is actually doing something a little less than a 360 degree turn.
This bit is based on a misunderstanding of the post, but I think it's true about the stance and might be useful:I don't have that tai chi chuan book with me, but I think you're talking about the instructions for changing from a stance like deng san bu 登山步 to sitting on crossed legs, which is how many but not all of the sitting on crossed legs stances happen in the form. In that case, the front foot goes from toed in slightly to pointing out- about a 90 degree turn. I don't know if that's clear. It's hard to find a video that shows it well- here's the best I could get. It's the transition from brush knee left, which ends in deng san bu into turn body/circle fist. http://www.youtube.com/watch?v=Q4P722tMotI#t=2m41sIncidentally, does anyone have links to other videos or materials from the Tai Chi Farm? I've heard great stories about it.
Edit: Oops, I have to correct myself. I found the page you're talking about online, and now I see it's talking about going from ma bu to sitting on crossed legs. I dunno. If you're turning to the right, the front foot is turning clockwise. When you're in the sitting on crossed legs stance after turning right out of ma bu, the foot is 90 degrees clockwise from the direction you're facing.