My reins always vary in lenght depending on the conditions. When you put a steady consistent pull on a horses mouth your not trying to force his head down you're only asking for him to put his head down. If he throws his head the pressure (which is not a lot) stays and when he breaks at the poll and softens then it goes away. It doesn't matter if my hands are high or low because he gets relief only when he gives the desired response. I would have done enough of this at the walk and trot that it would be a very short lesson at the canter. Incidently, and I'm not trying to offend, what is your experience (years, schooling ect.)? I do like the advice on lungeing I have often done that although I use a round pen and no lines.