It's not so much mind-to-mind communication, as it is "mind reading." How do you read a mind? Well, the mind expresses itself physically, in the body. Sigmund Frued wrote about how a person can lie with their words, but that their body will always tell the truth. When you mirror a person, and especially if you combine it with an active mental image of stepping into that person and being them, you'll know what they think and feel from their posture. If they also mirror you, and especially if they're willing to do the same "step into" as you do, then the raport effect can be something close to "Miss Cleo" level psychic communication. It's nothing supernatural, but it is something most people just don't consider as part of their reality.
