I play clarinet and am interested in learning to circular breathe.
- How is circular breathing done?
- Once I learn to do it, is it something that I would want to use throughout my playing or only for certain passages?
Circular breathing is a technique used to replenish your air supply while maintaining a tone on your instrument. The difficulty comes from coordinating the inhalation and exhalation, and maintaining a tone while the oral cavity is being utilized for air support.
The method is generally the same for all wind instruments, and you should start without the instrument present (like blowing against your hand or through a straw).
That's the basic technique of replenishing your air supply while maintaining an airstream out of your mouth. You will then need to refine your ability to transition from supporting normally, to filling the cheeks, to inhaling through the nose, to returning to supporting normally, all as smoothly as possible with a consistent airstream.
Once you've got that down, you can start adjusting the technique to fit your embouchure. Depending on the instrument or embouchure, you will likely have a lot less cheek room to work with than when you develop the technique without the instrument. This takes a lot of refinement, which is why circular breathing is such a seldom seen techniqe.
To answer the second part of your question, I would not consider this to be something you would use all the time. I see it most often used by jazz musicians when they want to look particularly impressive. Wynton Marsalis uses it multiple times in this extended solo phrase in Cherokee. Clark Terry was also known for this technique, and actually published a book on the subject, but good luck finding a copy! (It's out of print.)
It is also used occasionally in classical literature for musical reasons. (See Julian Bliss playing Mozart for the Queen.) Of course, most classical literature is written with breaths included, to kind of give the audience ear a chance to "breathe" along with the performer. On the flipside, if you played didgeridoo, circular breathing would be a pretty basic technique, and used quite often.