Just sitting on the horse's back causes tension. We want the horse to lift his back for healthy posture.
There are three ways a horse lifts his back
1.) Through lengthening the neck which causes the Nuchal ligament system to pull taught and lift the back
2.) By engaging the abdominal muscles
3.) By tucking the pelvis under
The horse can also lift his withers, lightening the forehand, and transferring weight to his hind legs.