This podcast explains what the non-manual features mean.
Non-manual features are movements of the head, shoulders, eyebrows, mouth, cheeks, changes in eye gaze, body shift and so on. All of these features are facial grammar and body grammar of sign language. For example, they can show differences in the types of sentences or the types of questions and can be used to describe the way how an action is done (whether with ease, effort, concentration or whatever). But there are facial expressions and body postures typically associated with emotions (sad, happy etc.) which are universal for all languages.