Cabeceo (a 'nod of the head')

Argentinian tango has an amazingly complicated process of negotiating and beginning a dance with a new partner.  The point of this negotiation is to give dance partners the opportunity to meet and engage new people subtly, while offering both the leader (typically male) and follower (typically female) the opportunity to gently decline, without embarrassing the "asker."

The literal translation of cabeceo is 'nod of the head'.  To initiate an invitation to dance, the man looks at the woman and indicates with a movement of his head that he would like to dance.  If she accepts the cabeceo, she will move towards him and they will tango; if she refuses she will look away.

A great example is shown on youtube:

http://www.youtube.com/watch?feature=player_embedded&v=gDbtXiB_b6s

The key to the usefulness of the cabeceo is its subtlety; It prevents an awkward situation and unpleasantness by using small and subtle gestures.  If the invitation is declined, the embarrassment is kept private between the invitor and invitee.  Pretty rad.