[1] Usually a very simple device, it normally provides an amplifier, and a loudspeaker, often battery-powered and packaged into a small, hand-held test probe.
An optional diode detector is usually also provided, allowing the detection of amplitude-modulated signals.
So long as the signal can be heard, the circuitry up to that point is (at least minimally) functional.
If the signal disappears, however, a fault can be assumed to be present in the stage of the circuit just passed.
For long pulse trains, a cyclic redundancy check may be calculated and displayed, giving the tech insight into the content of circuits that are switching rapidly.