To be fair most horse riders have little or no idea just how important it is to mind your seat. Most think of their seat as the balance point between their feet in the stirrups to keep them upright and in the saddle when riding.
But your seat means much more than you probably realize to your horse. Imagine carrying a heavy grocery bag in your left hand and the impact that has on your posture, balance, and movements. The weight in your left hand is counterbalanced by moving your shoulder to the right, and arching your back sideways, the same holds true for your horse.
If your weight is unevenly distributed with a bias in one direction your horse must counterbalance in some manner, which will impact its ability to move, either making it easier or more difficult.
Let’s use barrel racing as a great example, watch closely how the riders position themselves as they round the barrels. The best racers position themselves to help the horse stay in balance while they turn, most put a drag on their horses’ balance towards the barrel.
If you sit on the outside of a turn your horse can more easily maintain balance, maintaining a more upright posture – setting him up for a faster get away after the turn. So, when turning counterclockwise around a barrel, the rider should weight more on the right side of the saddle to avoid pulling the horse towards the barrel and off balance.
When turning a rollback on the wall, it helps if the rider’s shoulders are behind their hips, to shift the weight to the horse’s hind making it easier for the front feet to complete the turn – this is true for any turn on the haunches. Conversely, a turn on the forehand is helped by shifting the shoulders slightly ahead of the balance point, this lightens the hind making it easier to rotate through.
People will often pull their horse to a stop, which is a whole other crime, but in doing so they lean backwards putting more weight on the hind, and simultaneously lightening the front, which encourages the horse to throw its head and to rear up. This is an abbreviated introduction to weight and seat position, but I hope it helps someone.