How is body language linked with meditation success?

Body language (smiling) seems to be linked to meditation success as per Bhante Vimalaramsi: >smiling is an important aspect for the meditation. Learning to smile with mind and raising slightly the corners of the mouth helps mind to be observant, alert and agile. > >Source What are the mechanics behind this or how it works? ---------- In addition these talks through not related to make suggestion that there is a link between body language, smiling and our well being: - Your body language shapes who you are - The hidden power of smiling