hi wolfie, i'm a clarinet/sax player for a living, and although i haven't ever had to do it myself, and know many people who do use the technique on a regular basis (there was a chap from mexico city in london for a while - he had to - couldn't get through a phrase at that altitude without it apparently!
)
i''ll try to explain...
although the general opinion is that circular breathing is breathing in and out at the same time, this is not actually the case. what actually happens is that you store the air going out in your mouth (by puffing out your cheeks). whilst breathing in through your nose, you squeeze your cheeks together, so pushing the air you have stored in your mouth out, so keeping a continous air flow leaving your mouth. the best way is to practise with a straw in water.
it sounds confusing, but it's pretty simple. i don't know of anyone who's spent time trying and not been able to do it.
breath in normally
breath out, and store some excess air in mouth puffing out cheeks
whilst breathing in through your nose, bring cheeks back into normal position, pushing air out
continue breathing out until you need to do it again - store air in mouth...etc
hope this helps. good luck!
r2b2 - loads of jazzers do it:beer: