Well there's many at sessions that share the opinion that its a Didgereedon't. And as for circular breathing try a few excersises on circular letters like "O"
**BG**
I know a guy who makes them and even he hyperventilates playing them. He found you can play two on one pair of lips at the same time and get two notes but if you tried making one by joining two together via a Y shaped tube and blowing into the single end of the Y you get one note controlled by the combined length of the tubes. This is all to do with nodes anti nodes but is fairly logical. You can hum sing and do a lot of vocallising as well as playing as a low frequency trumpet. There is a one-handed guy in Sydney who plays one like a trombone, (plastic tubes sliding inside each other).