In NLP, there is a technique known as Mirroring. It is about mimicking other people’s behaviour.
One aspect of the mimicking is using body language. This is close to Covert Hypnosis that uses words to influence people’s unconscious mind. For mirroring, you use body language in a subtle way. This will make the other party more comfortable with you. It will not occur to that person that you are intentionally mirroring his movement, otherwise you get the opposite effect.
It is meant to influence relationship and communication.