One of the thing he was getting me to work on was keeping the knees out and really squatting down between my legs, rather than just sitting back. He explained that one of the keys was to keep the feet from collapsing to maintain the proper angle at the ankle. Interesting and challenging.
Kelly Starret explains similar principles and Dan John explains the goblet squat in the same terms:
....people discover at this moment ... a basic physiological fact: The legs are not stuck like stilts under the torso. Rather, the torso is slung between the legs.(There is another good goblet squat article here)
As you go down, leaning back with arms straight, you'll discover one of the true keys of lifting: you squat between your legs. You don't fold and unfold like an accordion; you sink between your legs. Don't just sit and read this. Get up and do it!
Kelly explains the issue of keeping a stable arch here: