Mindfulness then reminds the meditator to relax. After releasing the feeling (painful or pleasant sensation), and allowing it to be without trying to control it, there is a subtle, barely noticeable tension in the mind/body. This is why a tranquilizing step is emphasized. It’s easy.
Getting serious, tensing up, or frowning causes the mind to become heavy and one’s mindfulness to become dull. One’s insights become more difficult to see, thus slowing one’s understanding of Dharma (phenomena and the Doctrine).