The breath test can help diagnose whether a capsule's surfaces are contaminated with dirt, spit, dust and other foreign objects without having to take the mic apart.
A little background, for better understanding:
The operating principle of a capacitor (condenser) capsule requires that the two closely spaced plates of the capacitor remain extremely well insulated from each other, to hold the electrical charge that is applied (polarization voltage). If the capacitor discharges, due to an electrical path formed through dirt and moisture between the plates, thundering sounds and other odd noises can be heard.
In most condenser mics the isolation resistance measured between the plates is in excess of 10000000000 ohms (10 gigΩ). Imagine now that the surfaces of an abused (or incorrectly stored) capsule are covered with dried spit, dust, etc. All it takes is a little bit of moisture or excess humidity to form a conductive path between the plates- a path prepared by the layer of contamination.
To assess whether a capsule is contaminated, the breath test, then, will give you some clues of how clean or dirty the capsule is: Breathe closely into the mic's capsule area, as if trying to fog up a mirror, and listen what happens next:
The audio from a clean, dry capsule will disappear momentarily (due to the breath humidity bridging the two plates): for a brief second the capacitor electrically collapses (shorts out), and its two plates lose the isolation resistance in-between. As soon as the excess moisture has dissipated- typically within a second or two- the capsule's performance returns to normal.
Depending on the severity of capsule contamination, the breath test will trigger thunderous discharges, whistling, and other odd sounds, in addition to often lengthy disappearance of audio. If these discharge sounds continue for several seconds or longer, capsule restoration through removal of contamination from all high impedance components and wiring in the capsule area or, in severe cases, capsule replacement is required to return the mic to its full functionality.