We tend to mirror our partners and people we see in society as a part of integration and wanting to succeed and fit in. For example if you will have a conversation with a friend and lean forward he will probably lean forward as well and if you rub your neck he will also mimic the movement to some extent. If a partner strikes or works with a knife to slash our legs it may not be the best idea to stand there and try to work with just your arms to return the favor. Start by having a partner with a knife work on stabbing and cutting you with a knife and you fold forearm over forearm at your back and use your body first to move to survive. You can of course use your body to attack as well but keep your survival the number one priority. We keep the arms on the back in that way to help us understand the movements of the torso and if we have more than one partner we learn to free the eyes from focusing on something other than OUR surroundings.
Relax the face and people will be more relaxed around you.