The plank engages the muscles in the core (inc. lower back), hips and shoulders. Draw the navel in towards the spine and try to engage the core while preventing any sags in the core. Similarly you'd prefer not to have the butt too high in the air which deactivates the core to some extent. Using a dowel or broom stick ensures that the points of contact (head, upper back and glutes) remain stable with the core always engaged.