merely by copying their body language. If you think about it the process makes perfect sense. Why? If your brain sends messages to your body when you have a specific thought or feeling and those messages result in unambiguous body gestures or movements that relate to your mental experience, then by copying someone else's body language or body gestures, your body will relay similar messages back to your own brain. In doing so you will have a rough mental impression or feeling similar to theirs.
The process is actually quite amazing and with practice and experience you will find that the accuracy of those 'impressions' or 'feelings' can be surprising. But, beware, because it is not just a case of copying one or two selected gestures, movements or bodily positions. The more accurately you copy or mirror the entirety of the other person's posture and position the more connected your experience will become. You might like also to try mirroring their breathing, the location or direction of their eyes, as well as their facial expressions too. If you can calibrate these with real precision you may be amazed at the results, which can seem almost psychic in their accuracy.