I was thinking about the mouth during my last trail ride. Horse has always been very light, and our main task over the past several years has been teaching her to seek the bit, and go long and low with an open poll, stretching her topline. So she is a horse that just doesn’t have pulling, rooting, or hanging on the reins in her vocabulary. She would love to be a Western horse ridden from neck rein and seat aids, or voice command, with nothing ever touching her mouth.
Nevertheless, on the trails in the summer, she is capable of diving for tasty grass and leaves, and once she gets going on a dive, she is pretty much impossible to stop: she can just about wrench your arms out of their sockets. This from a horse that has an extremely light mouth in every other situation.
I can however completely keep her from diving for grass if I ride her with a bit of flexion both lateral and longitudinal in the poll. If I ride her shoulder-in, shoulder-fore, or even just with a little lateral flexion (what the French call flechi droit, bent but going straight), with her jaw mobiized, she simply cannot dive and get away from me. She will move past the tastiest tall grass waving at nose level without making a move. This is not a tight grip on the reins on my part. I am not holding her up there. She is as light as when we do these exercises in the ring. I think that what’s happening is that, in these “school” positions, she gives me her jaw and poll. In order to dive for grass, she needs to clench her jaw and neck, and brace against me.
So our trail rides are long and low, or loose rein, where there’s nothing much to eat, and shoulder-in past the tasty stuff.
Anyways, this has re-inforced my thoughts of a few days ago, that when we say “mouth” we are really talking about neck and jaw, not the actual bars and lips, in most cases. As far as the connection to the original question about how side-reins affect this, my preference would be to teach response to the reins from the saddle, with a rider with very good tact, feel and release, so the horse learned to follow a rider’s following rein. I would worry that the fixed aspect of the side reins would encourage leaning into the bit. Of course, as I also said earlier, I have never seen side-reins used by anyone who knew what they were doing.