The light beam from the emitter shines directly onto and past the string. The receiver reacts to this light beam and causes a small amount of electricity to generate within.   As the string moves,  it allows more or less of the light beam to be sent to the  receiver  where it is directly converted to audio signals.

As you know, each instrument has a unique sound imprint and cannot be duplicated exactly.  Your instrument has many overtones that are a combined product of  every part of  the instrument, and  therefore, no two instruments sound exactly alike.  This combined sound imprint is present on the string.  Pickups with magnetic fields will  alter the string's pure imprint by restricting some movement.  Any pickup that uses mechanical  coupling to the string will also alter the string's pure imprint.