In order for your horse to understand his bit cues or his leg and seat cues he has to be able to feel them. Contact is the amount of surface area where your body or your signal can be felt and the quantitative value of the pressure there.
Horses ridden in most English riding activities have constant “contact” in the mouth: that is, the rider keeps at least the slightest pressure on the bit at all times so that the horse feels the rider’s hands through the reins at all times.
Leg contact represents how much pressure the horse can feel from the rider’s legs. Good leg contact is critical to the horse and rider, as it tells the horse when to move left or right.
Seat contact represents whether the rider is upright or “on his pockets” leaning back. It can be very subtle, but the horse is very aware of where the rider’s fanny bones are pressing and how his weight is distributed.
In English riding, contact is enhanced by the fact that there is very little saddle or pad between the rider’s legs and the horse’s body. The leg you see is what the horse feels. That is also true of this “Cavalry Saddle”. Notice how there is almost nothing between the rider’s leg and the horse.
Western saddles have a disadvantage here. They generally have a lot of “stuff” between the rider and the horse: thick pads, long skirts, stirrup fenders, cinching, and riggings. During the introductory or teaching phase with a western saddle, it is often necessary to exaggerate body movement so that it is obvious what the seat or leg is asking.
I frequently start teaching cues with a bareback rider if the horse and rider are good enough for that. The horse has an easier time understanding the more precise communication, and the rider has an easier time finding his own heel and fanny bones as well as the right spot on the horse without all of the encumbrance of a bulky saddle.
Recent advances in western saddle design have not only brought new “flexing” trees, but saddles whose rigging “guts” have been lightened and stripped of non-essential materials to bring the rider’s legs into closer contact with the horse.