The trainer plays a note. You sing it back — any octave, any vowel. The engine measures where you actually landed, in cents. Five rounds, one grade.
Singing in tune is a feedback skill — you adjust to what you hear. The catch: most people can't hear their own 20-cent error, so they practice the error. Numbers break the loop. Land at −35¢ three rounds running and you've learned something specific: you undershoot, probably on the approach. Trained ears notice errors around 5–10 cents; scoring under 15¢ average puts you in solidly-in-tune territory for a normal human in a normal room.
Octaves are free here by design — matching a note in your register is the real skill (a bass matching a soprano's note an octave down is correct singing, not cheating). Find your register first with the vocal range test.