Physical control of the horse means go where I want him to go, how fast, how long and do the last thing I asked until I ask for something different. There are in essensce, only two cues (stop and go). Everything else is an aide (your feet, hands, stirrups, spurs, reins, weight, etc) which guide the horse once it is in motion.
I have witnessed a lot of trainers start at the back of the horse, yeilding his hindquarters. While this needs to be taught and you may be sucessful with domestic horse, it will get you hurt with a mustang this early in training. I begin in the front where the brain is located and fear orginates, by yeilding the forequarters. The first steps are to the side and away - not back and certainly not on top of me when we begin to lunge.
So we start from the front and move our way to the back and teach the go forward cue and then yeild the hindquarters. Everything initially taught on the ground with clear instructions.