The correct position in the saddle directly affects your ability to communicate with your horse. Good posture allows the rider to move as one with their horse and give the lightest aids with precision. Poor posture inhibits the horse’s movement and means that the rider’s aids are ill timed and unclear to the horse.
Sit on your seat bones with the pubic bone resting towards the twist of the saddle. The thighs should hang from the hip sockets, slightly forward, and the upper body balanced over the seat bones. Our spine has a natural “S” curve and this should be retained so that the lower back can move exactly with the horses back, otherwise the horse will be blocked.
The seat extends up through the lower back and if this does not move the seat is blocked. A supple lower back moves with the horse’s back, or you will block the horse’s back and the movement from the quarters.
Never support your position by hanging on the reins or gripping with the legs. When the position is right it should feel easy and connected. This position allows the rider to feel and direct the horse’s movement and not disrupt it.
The first picture below show a correct seat.
In the centre picture the rider has lost contact with the saddle and is gripping with the knees which is pushing the seat up towards the back of the saddle
The last picture shows how the rider has lost the seat forward, the seat is being held in the saddle by the thighs braced against the knee rolls and the hands are pulling back on the reins.
Diane Followell Classical Dressage Trainer