The uterus and intestines react to pressure from above and drop downward. With very poor posture, the organs can actually become kinked. Because the pelvic bones are aligned in a V-shape (narrow at the bottom, wide at the top), the levator ani muscle is pulled upward—like a trapeze—deeper into the body.
On the one hand, this strengthens the muscles; on the other hand, it pulls the abdominal organs higher into the abdominal cavity. As the upper edge of the pelvis expands, all the organs have more space.