The "man nod" is an unspoken greeting when you want to acknowledge someones presence, but do not wish to start a conversation with them. The only way to answer a man nod is with another man nod, only verbal part of a man nod is usually a grunt. The man nod is great for being sort of polite even when you dont want to speak to them at all. The man nod can also be done to non men by men, and also used on strangers if your a blokey bloke.
Steve was walking down the street when he saw Fred he knew from uni coming the other way, being a bloke Steve didnt want to bother with being social so he gave Fred a quick man nod, as Fred walked past he returned Steves man nod and carried on walking.