If the human brain is exposed to one frequency in the left ear which is 10 Hertz below another audible frequency played in the right ear, rather than hearing either of the two audible frequencies, the brain chooses to “hear” the difference between them, the “beat” frequency.
Since the Theta level is associated with sleep, the subject concerned may progress from a fully awake to a sleep state (provided that he does not consciously resist) as the brain strives to entrain its wave frequency output with the one which the person hears.