This is one of the many exercises that Mary Wanless presents in dismounted workshops. Many riders have not calibrated their bodies and are unaware that their perception might not match reality!
Mary presents Rider Biomechanics in bite-size chunks. First, she covers some essential material to help you become an effective learner. Next, she guides you to develop a stronger and more stable position in the saddle so that you can move with your horse. Then, when you are aligned and balanced, you can progress to using your body to be a positive influence on your horse.
The way you perceive your body has a big effect on the way you sit; the way you sit has a profound influence on your horse, and for each little change that you make, he changes, too. Learn how this works, and you and your horse will both be winners!