When a string is plucked and the amount of energy generated by the pickup that causes the input stage of an amplifier to amplify. This overdrive is often associated with sustain or how long a note will continue to vibrate for the desired duration. Sustain is also associated with the length of time a note will vibrate and until muted by the palm of fingers. Feedback from volume will also cause the note to sustain and much depends on some of the examples below.

Design of pickup and physical construction.
Gauge of magnet wire, number of turns, number of turns per layer, number of layers.
Magnetic path, magnet material, gauss, flux field, pole piece design and shape.
String gauge, string material, pickup distance from strings, pickup distance from bridge or neck.
Instrument material, weight, density, neck and body connection, hardware such as bridge, tail piece, nut, frets, fingerboard and keys.
Amplifier, pre-amps, cables, volume and tone controls etc.