h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

There is a thing in your throat called your glottis which is the bit most likely to interrupt a smooth flow when circular breathing. The glottis closes the air passage, which is pretty handy when eating. It is not so handy in circular breathing because it tends naturally to punctuate the airflow.

If you inhale then hold your breath while letting your ribs fall as though you were breathing out, then your glottis is the thing stopping the air coming out of your lungs.

If, on the other hand, you hold your breath by holding your ribs up and your chest expanded, you don't have to close your glottis. If you can feel the difference, you know what you are trying to smooth out while circular breathing. How you actually manage this is rather more difficult to describe, but easy to do now you know what a closed glottis feels like!