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How does the mouth change the sound of a mouth bow?

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Koto — stringed instrument A Japanese musician plucking the strings of a koto with the right hand to generate a pitch and pressing the strings with the left hand to alter the tone.

On a mouth bow, the player uses their mouth as a resonator to amplify the sound of the string. By partially inserting the instrument into their mouth and striking or plucking the string, the player can produce a melody.123

The mouth changes the sound by varying the size and shape of the oral cavity. These physical adjustments allow the player to selectively emphasize specific overtones produced by the vibrating string. Because the fundamental note of the string is often weak, isolating and reinforcing these overtones through the changing shape of the mouth cavity creates a clearly perceptible, albeit quiet, melody. This process is similar to how other resonators alter the quality of a tone by changing the relative intensities of overtones.314