A mantra cry is different from proper crying.
Every LO has a different mantra cry so it is hard to describe but I'd say it is a less intense sound, more repetitive (like a mantra), more of a humming, no tears, no escalating (if it does escalate or start to produce tears then it's changed to a real cry, you need to respond) and for the most part doesn't tend to sound too sad.
I do remember though a certain phase mine went through when his mantra DID sound sad I I started to jump in too early, when I went in the room I realised no tears and from the repetition of sound realised it was a mantra, also although it sounded sad it didn't escalate.
I don't know how old your LO is but if you are in the room the chances are he knows you are there so he will not feel abandoned. You do not need to pick up through a mantra, you should let him self settle as much as possible.
A mantra can also end with a final yelp/cry out/scream. This will be the one that gets you jumping from your sofa and running to his room to reassure him that mummy will always return - only to discover that actually he nodded off a split second after the yelp. I think Tracy described it in the book, a last ditch attempt to stay awake before succumbing to dream land or something.
hope this helps
