Now, the nod is very wonderful gesture. It can be used to pretend you are understanding, or to show that you agree, or the most complex - to acknowledge someone on the road.
Now this acknowledgement nod is tricky business. It normally is just a quick bob of the head with it ending, and this is very important, back where it started.
The quick bob with no smile is normally reserved for acquaintances who you really don't want to stop and speak to, or worse, who's name you don't remember. This mostly says "I see you. I don't want to stop and talk now. I have acknowledged you. Now lets move on"
The other nod is more of a long upward movement of the head and a then a very small downward movement. It is essential in this move that your head remained slightly raised till you pass out of sight. This nod, commonly used in metros, is normally accompanied by a slight upward twist of the lips on one side (colloquially called a grin).
This is used to acknowledge peers/ friends who are at some distance from you. This is an alternative to screaming out obscenities over that distance, that you normally use at nicknames for that friend.
This nod is in recent times is used in accompaniment with a cell phone. Thus you don't stop talking on the phone but just do the upward nod while talking as if to say " I have a life. I cannot spend it stopping, to talk to you."
A modification to the above nod, is the eyebrow nod. This is only a very slight shift in the angle of your double chin, but who's whole identity rests on the eyebrows. The idea is to raise one eyebrow as a gesture of acknowledgement.
This has a mischievous connotation to it and is normally used in situations where you don't want to disturb the acquaintance.
Now the most delicate of Nods is the good-bye nod. This is used in situations where you make the fatal mistake of stopping to talk to an acquaintance on the road.
Once the regulatory 1.25 mins are done and you have done the how are you, how is boyfriend/ girlfriend, husband, wife, parents, dog, imaginary pet; the "hows work going", you start hinting that you are on your way to some place.
Now most road acquaintances choose to ignore these signals and instead proceed to a long (and supposedly interesting) story of the colour of the sore on the neighbour's cats leg.
Now it's very essential to recognize a logical break in the story (this is what makes a true artiste) and use this pause for breath to remind them that " I have to go. I'm really late." and of course throw the "let's catch up sometime soon" line... and now comes the clincher. You must do the good-bye nod.
This is a quick sharp military style downward nod that stops with a sharp jerk. This says " I have to go now! NOW! NOW! NOW!"
Now some people not schooled in the art of nodding, do not recognize this need of yours to move. And will continue to inunciate the finer points of checking fungus on a sore.
Now this is when the vigourous nodding of the head starts and you start walking backwards.
It is also considered a matter of finesse if you take a first step to turn around during the backward walking trick.
This says " I have left."