From cabeza; head:
Traditional technique for selecting dance partners from a distance at the milongas in Buenos Aires by using eye contact and head movements.
See also Codigos
At the milongas in Buenos Aires, often not a word is spoken in asking someone for a dance. A man will look around the room and try to make eye contact with a woman. Also, a woman can initiate the eye contact as well. If she does not want to dance with him, she will avert her gaze. If, however, she wants to dance with him, she will make eye contact; he will then slightly nod his head in the direction of the dance floor. If she has decided to dance with him she will nod yes. Only at this point would a man go to a woman’s table and escort her to the floor. This set of conventions serves a myriad of purposes. First: it prevents women from feeling obligated to dance with just any man who comes to her table and asks her to dance. Second: men are kept from looking foolish by going to a table and being refused a dance. Third, if for any reason something comes up, or anyone changes his or her mind, no one else need ever know.
(from tangonadamas.com (http://www.tangonadamas.com/abouttango.cfm))
The cabeceo takes practice. It takes right timing and a sharp eye. One can avoid an awkward situation by learning patience. Give the man time walk across the floor. He will make eye contact with you to confirm his signal.
(from totango.net (http://totango.net/cabeceo.html))
Their research showed that the traditional cabeceo was developed for important sociological reasons—the protection and preservation of male ego and female vulnerability. How else can one guarantee a man’s protection from public ridicule if he is to walk across a crowded dance floor to ask a woman to dance risking potential refusal? How else does one protect the tender nature of a woman who may not have the strength of character to refuse a dance with a man she does not want? The answer is the cabeceo—the mantle of protection that allows both genders a choice!
(from tangonoticias.com (http://www.tangonoticias.com/articles/a2004/nov/eyes.html))