I just watched a couple videos from Boston Dynamics and it looks like they changed the leg design since the last I saw. Now the knee can bend either direction, and from the product features video now it makes sense. Humans have to do a lot of turning of their whole body just to face what they're looking at, but with these reversible knees, now Atlas can go back and forth between two points and only turn their head and their arms. It's for efficiency and it's fascinating!