Why a meaningless sound works
Campbell's default is not the breath. It is a mantra — and his account of why a mantra works is entirely mechanical, with nothing mystical in it at all. You stop the incessant, self-referential chatter of the mind, he writes, by filling it up with something less distracting, less self-focused and less obsessively driven. While the mind is busy with non-operative busy-work, you can experience the still centre of your being.
The crucial property is meaninglessness. There is no way for you to be judgmental about a nonsense sound. No way to analyse it. So the analysis and the judgments and the ego attachments have nothing to grip, and they fall away.
He gives his own examples and tells you to mix the syllables freely: sehr-ring, da-room, ra-zing, ca-ouhn. Two syllables, no meaning, ending in something that rings or hums. And a warning aimed squarely at a certain kind of person: obsessive-compulsive types should take care not to get wrapped around the axle trying to find the best one, because any will do.
He also generalises it across the senses, because not everyone thinks in words. There is a visual version — a football that slowly becomes simple shapes — and a tactile one, running your hands over a heavy velvet coat. Pick whichever matches how you take things in. Breath is fine too; he names it in a Q&A as one ordinary option among many, on exactly the same grounds. It is boring, and there is nothing in it to judge.