During the changing of the guard, I have always noticed the relief commander either relaxes their jaw before most sentences they say, or actively juts it out for every sentence (depending on the individual commander). Being one of the more enshrined rituals in the United States military, I'm assuming there's a symbolic meaning to everything. What is the symbolism/practical purpose of this particular motion?
EDIT: one good example occurs at 4:55 or so when the relief commander says "forward, march." The camera shifts to see his face just before "march" as the jaw movement is noticeable then. Another good timestamp is around 6:15