Master Mantak Chia is a popular Taoist chi-kung guru in the west, most popularized by his interpretation/adaption of sexual chi-kung into new age concepts like "multi-orgasmic man" (WOW!). Anyway, I have expressed my views on sexual chi-kung previously (and shall add more in future), but this post is about Chia's smiling meditation.
Meditation can be classified into two categories (there are of course other valid classifications too). The first is mind-only visualization of a calming scene with a, often time, happy mood. The other is mind-body in nature, whereby the mind is to focus itself on internal chi (body) generation process. Some soothing music will probably be beneficial to the mind-only type, and typically, a practitioner will smile and enjoy the whole calming and relaxing process of mind-only practice.
On the other hand, if one's mind is focused on one's chi generation process, one's mind will, assuming successful, be totally focused on this inner, purely physical, event and the mind simply can't entertain other thoughts. And the facial expression of the practitioner will signify whether or not he or she has arrived at this highly focused state. A professional meditation (standing and seated) coach will use this as a guide-line to check the progress of his students.
This is the state-of-the-art of meditative coach in contemporary China. And teachers here have a word for it "打楞", and can be translated as "a stupefied look". It signifies the practitioner to be "in the zone". Needless to say, this is the way I teach my students, instead of asking them to do any form of inner smile.