What technique is especially associated with the didgeridoo and lets a player sustain a continuous tone?

Circular breathing is the technique associated with the didgeridoo that lets a player sustain a continuous tone.

The performer stores a small amount of air in the mouth, uses the cheeks to push that air outward, and simultaneously inhales through the nose. The mouth then returns to normal exhalation before the stored air runs out. With practice, the transition can be nearly seamless.

Didgeridoo traditions among Aboriginal Australians use continuous airflow as part of extended performance, along with rhythmic vocal effects and changes in tongue and throat position. Circular breathing is not limited to this instrument; some oboe, bassoon, saxophone, and brass players also use it for endurance or special effects.

The technique does not mean that the lungs hold an endless supply of air. It is a coordination method that briefly overlaps oral-air pressure and nasal inhalation while the instrument continues sounding.