Real good boxers are very aware of the opponent's feet. I started thinking that a long time ago watching Henry Armstrong fight, with his head down, the way he rolled inside punches to punch back. It seemed to me that he was keying off their feet to know what punch was coming. That little turn of weight will give it away, the feet shift, right?
Around that time I got to talking with a guy that ran a gym near where I lived. He said he'd had hundreds of fights in the 20s and 30s, and he looked it, though I've not found him on boxrec...But he showed me a few things that are based off knowing right where a guy's feet are at a particular moment. How you can trap him with your foot and line him up for a punch, like that, or turn him to where he can't punch back without picking up his front foot. Knowing things like that, about an opponent's feet, affects what punches you throw at him.