Side view of a jack

You can see the damper felt, and the plectrum (in this case made of Delrin, but traditionally of condor [now illegal of course] or crow's quill). When the jack is at rest, the string would pass a millimetre or so above the plectrum, with the damper resting on it. On the side of the jack you can see a small hole through which a pin is pushed, to allow the tongue that holds the plectrum to pivot