The position of pelvis has an influence on the movement state. If the pelvis is not in the right position, it will make unnecessary compensation for the joints above or below your body chain. When exercising, the fitness instructor presses the position of the pelvis because the position of the pelvis has an influence on the exercise state. When the pelvis leans forward, your rear thigh muscles and abdominal muscles will stretch and your lumbar spine will stretch (the front arch of your body).