The key is balance, or keeping the head in the center of the body throughout the swing and rotating around a stationary axis. Even hitters that make contact with their back foot slightly off the ground have resisted their weight moving forward by keeping their front leg solid and not collapsing the front knee. I am not so concerned with what the back foot does during the swing, so long as the back leg rotates and the front leg remains solid (some front foot rotation is fine, just no bending of the front knee). I am mostly concerned with keeping the head centered and rotating around a stationary axis when I teach.