The stomach should move front to back, in a relaxed manner, with the horse’s movement. How is your horse affected by a tense stomach? A tense stomach area will result in the rider bouncing more on the horse’s back due to the stiff seat and back. The more the rider bounces, the more the horse is affected by picking up his head and hollowing his back. The rider can feel the horse becoming quicker and bouncier. This can become a difficult cycle to break because once the horse starts to hollow his back, the rider stiffens the stomach area even more. To add to this picture, the rider usually takes a hold of the reins and starts pulling to try to slow the horse down, which causes the horse to stiffen and pull against the bridle even more. It’s a vicious cycle.
You cannot ride with a relaxed stomach if you arch your back. Riding with an arched back creates several problems. It tips the pelvis forward, stiffens the entire upper torso and puts the rider’s leg in an incorrect position. Think about it, your stomach is your shock absorber. Assuming that you like your horse, you want to have a shock absorber of Cadillac or a Lincoln Continental. When a rider arches their back and locks their abdomen, they will have the shock absorber of a Mack truck on a bumpy road. Be a Cadillac, not a Mack truck.
Relax your back, roll your pelvis under to enable you to sit on your Ws (the pockets of your jeans on your behind, mine are always Wranglers), and relax your back. That will give you the Cadillac stomach shock absorber!