circular breathing

I haven't mastered the technique when drawing, but I can do it blowing (I actually learned it when playing trumpet in high school).
Basically, while you are blowing (from your lungs), you allow some "extra" air to accumulate in your mouth/cheeks before you inhale. Then, as you inhale (or, starting just before you inhale), you push the accumulated air out through the harp using your cheek/tongue muscles. The trick is doing it smoothly, without a noticeable change in the volume when you make the switch from lung air to mouth air and back again.
Drawing should work the same, but it seems harder to me, and I can't do it while holding a bend at all, at least partly because I can't use my tongue as effectively to draw air into my mouth when when I'm trying to hold it in the right position for the draw. And it's just harder to draw air into the mouth using cheek muscles than it it to empty air ("blow") from cheeks that have been inflated somewhat prior to taking the breath. I probably need to practice it a lot more, but I'm not sure how much practical use I can get out of it at my stage.
I can impress kids by holding a blow note FOREVER. . Don't know about audiences yet, though.

Try this. WITHOUT breathing, force air into and out of your mouth, using your cheeks/tongue. Push it in and out fast, then breathe through your nose while you're doing it. This will prove to your brain that those two actions can occur independently. After that, it's just a matter of practicing to get control.