The Toggle Tone button toggles between the listening mode and the tone-generating mode.
Listening mode is the usual tuning mode. Tone-generating mode is used mostly for troubleshooting, although it may also be useful during stringing and chipping.
In the listening mode, the software listens to the sound from the microphone and generates a sweeping tone to indicate the tuning error of the sound it hears.
In the tone generating mode, the signal from the microphone is ignored, and the software just generates a fixed tone at the desired pitch of the selected note. The pitch of the generated tone uses the same calculated target pitch that is used in listening mode. It takes into account the stretch, the partial, and the current offset. The generated tone is at exactly the same pitch that would indicate as perfectly in tune in listening mode.