The hands (and other awkward matters in equitation)

Reins are not a mean to impose your will by strenght, but a mean of transmission. Using a pre-established code, the rider can « tell » his horse what he expects from him. The rein is essentially a communication tool, not a pulling tool.
We have to assume (…) that you certainly must not « hold » a rein, with all that it implies. The truth is that a rein is very close to a music instrument, it is a piano, a violin, a harp… One plays music on a rein, one press on the rein as on a piano key resonating in the mental of the horse, like a piano key resonate musically. One makes the rein vibrate, gently stroking the mouth of the horse. By mere variations of angle, one should be able to change the horses weights. By a tiny vibration, one mobilizes the jaw. (p.23)

One of the most important mecanical reality in horse riding is that the horse has to move as if he was moving freely, without any contraction, so that he can be able to use all his muscular means.
But, if there is weight in the reins, if the horse is « weighting » on the hand, if there is a tension between the horse mouth hand the rider’s hands, this tension occurs in the same proportion in the rider’s hand and in the horse mouth. And if there is a tension in the mouth, the horse has to balance it out by a proportional tension in the muscles of his neck. This contraction can be light, medium or high, depending on the actual tension. If we want to ride in a really very good fashion, our horse has to be entirely supple, there should be no weight.
(p. 26-27).

                                                       From Les Mains et autres non-dits de l'équitation, Belin, France, 2006.