When a kiddo is first learning to roll they often draw their legs up into the air and use the momentum as they fall to one side to learn to roll onto their tummy. This technique keeps the trunk (shoulders through hips) moving together; we call this log rolling. Later on kiddos develop a more advance way of rolling by rotating their body in order to roll. You can see that once this kiddo rolls onto his side his mom rolls the ball out of his way to encourage him to reach for the ball in order to bring his trunk over his shoulder and be more balanced on his tummy to complete the roll.