Kharaj is the low octave — mandra saptak — and kharaj sadhana is the practice of holding one note in it against a drone until the breath runs out. Then again. There is nothing else to it and it is the practice almost everyone skips.
It is where the voice learns to sit still. A voice that cannot sit still down here will wobble everywhere else, in a way no amount of fast work hides. Fifteen minutes before anything else, every day, does more for a singer than an hour of ambitious practice twice a week.
Do not push for the note. Set Sa where your voice already lives, sound the drone, and let the low Sa arrive underneath it. A Sa pitched too high to sustain a low octave is the commonest beginner's mistake, and it is usually vanity.
Listen for the beating between your voice and the drone: a slow pulse when you are close, slowing further as you approach, gone when you are on the note. The ear will tell you before any needle does.
The needle is only there to confirm what your ear already said. A student who watches the screen and not the drone learns to tune to a picture, which is of no use the moment the picture is taken away.
Your Sa is yours to set. Sing comfortably low, tap Match my voice, and the target becomes exactly the note you are singing — which is what kharaj sadhana does anyway: the voice sets Sa, not a keyboard. Or pick a named key by hand.
The counter is unbroken time within half a semitone of that note. It resets when you leave the note, and a brief dropout is forgiven. Alongside it the screen shows how far off you are in cents and which way to move: a hundred cents is a semitone, and an untrained hum wanders further than most people expect.
A wander of a hundred and forty cents is not a held note and the counter will not call it one. Steadying it is the exercise.
Use headphones if you want the drone on. Through a phone's speaker the microphone hears the drone as well as you, and the drone's Sa is exactly an octave above the note you are holding — which the detector will read as a perfect low Sa that nobody sang.
Nothing you sing is recorded, stored or sent anywhere. The measurement happens in your browser and is thrown away frame by frame.