[v15w]

Pitch is an estimate based on sampled audio. Sometimes, it is not always accurately detected. Here, we have an additional visual cue that produces a stable/stationary waveform when the detected pitch is in tune with the selected reference note. A plucked tone has a slight variation in pitch from start to end; it rises and falls slightly. You can see this in the visual display. This would be very useful for fretting work, where each fret's tone is expected to have similar pitch variation.

[Ruarl]

I see. Was there an example of that in the video? Perhaps I missed it.

In what way does the visualisation vary when the note is not in tune? How does the visualisation indicate flat or sharp?

[v15w]

No. When sharp even by 0.5Hz the waveform rotates right and left when flat.