What is the cabeceo?
The cabeceo (meaning head nod/gesture) is an artform rather than a behaviour protocol. Just as a conversation has certain conventions but is quite subjective; requiring contextual understanding, sensitivity to the situation, the other person and a willingness to engage, so too the cabeceo is a non-verbal conversation. We get better and more natural at it over time and with practice as long as we have an interest in developing it.
Like a conversation (and much to most people's misunderstanding), the cabeceo is not one sided, i.e. instigated always by the leader or by the follower. It's much more fluid and starts with ordinary eye contact. As we know from our non-tango life, holding a gaze implies interest, so it is with the cabeceo. Once interest is established, in most cases the male leader (because Tango was situated in a machismo culture and habits continue) 'asks' the lady if she wants to dance by a small head movement or raising of an eyebrow and she either ignores his gesture or nods or smiles to accept. Of course, when people know each other already, or even sometimes when they don't, the cabeceo can be much more playful and the so called 'format' can change. For example, once eye contact is established, the follower may smile cheekily or innocently or sultrily, or just move her eyes or an eyebrow which sets the tone for the interaction and is her way of 'asking' the leader to dance. He might respond by a similar gesture as if to say, 'are you asking me to ask you to dance?' and she may respond with another little gesture as if to say 'maybe, why don't you try and see' or 'yes' or 'of course, I'd love to'... and each gesture would of course look very different and no one person is in charge of the asking.
So you see, the 'rules' for the cabeceo are as fixed as the rules for flirting, flirting without words. If you're in a pub late at night you might behave one way, if in a posh formal looking bar, you might behave another, if you're at a friend's child's christening it might be quite different to flirting with someone at a singles night and if you're a woman who feels strongly about taking charge of her night as opposed to being wooed you'd behave differently again... The whole thing is so personal and contextual, the only thing certain about the cabeceo is that it's a reference to asking without words with subtle facial expressions and head gestures. How you use it, and how successful you are at it, therein lies the artform.