What it is for
A held tone gives your voice a fixed thing to measure against, which converts a vague impression into a claim you can check. Sing a distance above the drone, then let the drill play the same distance and find out whether you moved as far as you thought. That single loop — commit with the voice, then verify — is what singing it back is about, and it is the fastest correction available in ear training.
How to use it
Start the tone, hum along with it until you are locked to it, then move to the distance you are working on and hold that instead. Come back to the drone every few attempts: drifting slowly away from a reference without noticing is the normal failure, and hearing yourself land back on it exactly is the correction. Nothing here is transposed or randomised — that is what the drills are for.