How to work with fine-tuning training
The module plays two notes, one of them slightly out of tune, and asks whether the second is too high or too low. On the first levels the deviations are obvious; later they shrink to a few cents.
Listen for the beating, not for the pitch. When two almost identical notes sound together, a slow pulsing of the volume appears. The closer the notes, the slower the pulsing; at exact agreement it disappears. That beating is an objective signal and far more reliable than the feeling that something sounds "somehow wrong".
Practise on headphones in a quiet room at moderate volume. Fine hearing is sensitive to background noise, and an excessive level tires the ear within minutes.
Cents, beating and temperament
Pitch differences are measured in cents: a semitone has one hundred, an octave twelve hundred. Trained listeners detect deviations from about five to ten cents, and with simultaneous notes even finer, because beating comes to their aid.
The beat frequency equals the difference between the two frequencies. Sound 440 and 442 hertz together and the tone pulses twice per second. Piano tuners work with exactly this: they count beats per second and thereby produce equal temperament, in which every fifth is very slightly narrow.
Concert A today usually sits at 440 hertz, many orchestras tune to 442 or 443, and historical ensembles to 415. That number is a convention, not a constant of nature. What matters for playing together is not the absolute value but that everyone uses the same one.
From practice
Almost all guitar students rely entirely on a tuner and never listen. That backfires the moment a string drops during playing or a fret intonates slightly off. So tune without a device regularly, with a check afterwards: listen first, then verify.
A good exercise on the instrument is tuning via harmonics or via the fifth fret. You play the same note on two strings and listen for the beating. When it disappears, you are in tune. It is the same principle as this module, only with real strings.
For singers and wind players fine hearing matters even more, because they produce the pitch themselves. Anyone sitting high or low in an ensemble usually does not hear it in their own sound, only in the whole. This module trains exactly the attention that requires.
Common mistakes
The most frequent mistake is confusing too high with too low. Many people reliably hear that something is off but assign the direction wrongly. It helps to sing the reference note internally and feel whether the second note pushes above or below it.
The second mistake is comparing from memory. If too much time passes between the two notes, the first one fades. Use the replay function and hear them immediately after each other.
The third mistake is practising too much. Fine hearing tires faster than any other ear-training task; after about ten minutes the hit rate drops measurably. Short sessions, done regularly, are without exception the better strategy here.