Micro expressions are involuntary facial expressions lasting for half a second or less. These facial expressions mirror a person's true emotions and so provide a window into a person's feelings in response to a statement or question. Particular facial expressions correspond to particular emotions and the mappings are universal across cultures. For example, when someone raises one side of his upper lip, this expression denotes a feeling of contempt by the expresser toward someone or something that he feels he is superior to, in some way.
Learning micro expressions and body language can serve as a very useful tool in analyzing footage of GB members to find out how they really feel about what they're saying or questions posed to them. For those still in, micro expressions given by C.O.s and elders who are on stage giving talks; or even by elders meeting with you on a shepherding visit can help you to determine what an elder really thinks of a question posed to him and if he's lying about a statement being made. It can help you identify others who may be secretly awake. For example, just ask a JW a GB-praising question or some other question that only a sleeping JW would welcome but which will make an awake JW feel contempt, and see what micro expression the JW gives.
Being able to read micro expressions can also be very useful in other areas of life such as in relationships, negotiations, on the job, job interviews, etc.