I always test horses after I've taught them everything by taking them outside to places they haven't been and then seeing how much they are really paying attention to me. I always lead a horse behind my shoulder and if I stop suddenly, sometimes I catch them off guard and they get a little ahead of me, but I expect them to automatically correct themselves by stepping backwards to get back behind my shoulder. I tested this with a (formerly) lazy gelding, he walked past my shoulder when I stopped but he didn't correct himself because he was distracted, so I turned the opposite direction and walked, and he hit the end of the lead rope and got yanked. It only took that one test for him to realize "oops... gotta pay attention." But I find it better to get a horse to stay behind me by letting him make the choice, not by pulling him back by the halter to get behind me. There's less resistance that way, and it encourages the horse to pay attention and do his job which is to follow me, not just walk around absentmindedly.