The subject’s shoulders will always move either in preparation or on delivery of an attack.
Try this simple exercise: Stand in front of a mirror and try to punch, kick or draw a weapon with out moving your shoulders.
This is why some street fighters will try and get in your face in order to use the head-butt and mask any shoulder movement.
There are three key tips here:
1.) Never let anyone get within head-butting range.
2.) Always keep your hands above your waist in anticipation of attack (thinker pose)
3.) Always look across the subject’s shoulder girdle for tell tale signs of movement.