Good article. 
Its called mirroring and it helps create instant rapport and connection as well as comfort. We all mirror its an instinctual automatic response seen even in babies. You visibly see it occurring when you yawn and those around start to yawn as well. This is your automatic mirroring response.
Think about it… when your upset or anxious your body tends to show it. You avoid eye contact, look down more, fidget, use closed protective body positions. What is the mirroring response going to be ? They start to feel uneasy with you.
Approaching someone who is displaying comfortable at ease body language you want to match that to connect. The “I’m just like you signal”
However suppose your looking to approach someone who is anxious and showing it with their body language. You do not want to match those movements. It will just escalate the uneasiness. In this situation you know you have to put this person at ease first then mirror them. You use non threatening gentle body language. When they begin to show signs of comfort you mirror those signs.
Tisha