It is the unconscious power that one has over somebody else's will. There is submitting charisma and submissive charisma.
It comes from the facial features. Any facial feature that is slightly exaggerated or asymmetrical, thus being perceived by the observer as viewed from an angle, usually from below.
A big chin is one of the first qualities that gives the perception of viewing a face from below. A child being held is having a point of view that gives the impression of a bigger chin of a person that is otherwise "normal".
A mustache gives the impression of a bigger chin by fooling the perception into thinking the mouth is higher on the face. A beard actually enlarges the viewing perception of the chin. An asymmetrical eye or eyebrow and a big chin gives the impression of viewing the face from an angle below.
All of the above gives the charismatic person the power of the mother that holds her child from the mere fact that some facial features of the charismatic leader are perceived as seen from the position of the child being held.
It also comes from attitude. A calm and reassuring attitude is the second condition of the charismatic "leader". I put here the word leader between quotes because a charismatic person needs more qualities to be a leader.
There are many people whose charismattic qualities are nullified by the lack of reassuring attitude.
This is also obvious if we refer to the same proposed theory, of the submission of a person by seeing a face as a child being held by mother.
But most of the charismatic people realize sometimes in life that he/her posses this quality and fall into the temptation of using and end up depending on it.
When people with little education try and exercise their charismatic qualities over those with more education sometimes are being rejected by the failed subjects and often labeled as trying to patronize or having a patronizing attitude.
The opposite is also true, men looking for women prefer a smaller chin or elevated eyebrows as a person would appear to have if seen from above.
Usually what happens nowadays, innate charismatic persons are being hired by others, usually becoming "politicians" with the only purpose of projecting an image of a leader and news breaker to the submitted crowds. (There is an interesting discussion here, as real politicians could be in fact all hired charismatic persons, and in rarer instances they might have learned to use their charisma for themselves)
I put the picture below as an example as i wrote this with this face in my mind. I read so many negative articles about this gentleman that is impossible for me not believe that his sole quality for the proposed function is charisma.
